The Boeing B-47 Stratojet jet bomber was a medium-range and medium-size bomber capable of flying at high subsonic speeds and primarily designed for penetrating the airspace of the Soviet Union. A major innovation in post-World War II combat jet design, it helped lead to the development of modern jet airliners. While the B-47 never saw major combat use, it remained a mainstay of the U.S. Air Force's Strategic Air Command (SAC) during the 1950s and early 1960s.
By this time, the war in Europe was obviously winding to a close. General Henry H. "Hap" Arnold, head of the USAAF, asked the prestigious expatriate Hungarian aerodynamicist Theodore von Kármán, of the California Institute of Technology, to form up a committee of American scientists to go to Europe and examine captured German technology. The result was the "Scientific Advisory Group". One of the members was Boeing's chief aerodynamicist, George Schairer. During his visit to Germany, Schairer examined data obtained by German aircraft manufacturers on the advantages of swept wings, and became so convinced of the merits of such a design that in May, 1945 he wrote a letter to Boeing management suggesting the matter be investigated.
Meanwhile, the USAAF had awarded study contracts to all three aircraft manufacturers working on the jet bomber project, as well as to Martin, which had also decided to join the fray. All of the competing bombers, including the North American B-45, Convair XB-46 and Martin XB-48 would have conventional straight wings with four to six engines, and would lack the performance of the swept wing B-47.
The NACA wind tunnel tests showed that the Boeing model suffered from excessive drag. Boeing engineers then tried a revised design, the Model 432, with the four engines buried in the forward fuselage, but although it had some structural advantages there was little effect on drag. At this point Boeing engineers turned to the German swept-wing data. A little design work by Boeing aerodynamicist Vic Ganzer led to an optimum sweepback of 35 degrees.
Boeing modified the Model 432 design with a swept wings and tail, resulting in the Model 448, which was presented to the USAAF in September 1946. The Model 448 retained the four TG-180 engines in the forward fuselage and, at the instigation of project manager George Martin, added two more TG-180s buried in the rear fuselage to provide greater range and performance.
Boeing submitted the Model 448 to the USAAF, only to have it rejected immediately. The Air Force strongly disliked fitting the engines in the fuselage, since that made engine fire or disintegration catastrophic. The engines would have to be moved back out on the wings.
That led straight back to the drag problem, but the engineering team came up with a clean, elegant solution, with the engines in streamlined pods attached to the wings. This innovation led to the next iteration, the Model 450, which featured two TG-180s in a single pod mounted on a pylon about a third of the way outboard on each wing, plus another engine slung from the wingtip.
The Air Force liked the new configuration, and so the Boeing team continued to refine it. One problem was landing gear. There was no space for landing gear in the thin wings, and trying to put conventional tricycle landing gear in the fuselage would have ruined the aircraft's streamlining and degraded its performance. Furthermore, the USAAF was now also insisting that the bomber be able to carry an atomic bomb. As nuclear weapons were very large at the time, that meant a long bomb bay, further limiting space for landing gear.
The solution was a "bicycle" landing gear configuration, with the two main gear assemblies arranged in a tandem, not a side by side, configuration. Outrigger landing gear was to be fitted to the inboard engine pods. The concept had already been tested on a modified Martin B-26 Marauder aircraft.
However, bicycle landing gear made it difficult for a pilot to "rotate" an aircraft into a nose-up position for takeoff. Again, the solution was simple: the landing gear was designed so that the nose-up position was the default. This little change would have a very pleasing effect on an aircraft that was already shaping up to be very elegant, giving the machine the appearance of being ready to leap into the air even when it was sitting still.
There were some other tweaks to the design, such as a wingtip extension to improve range. This had the effect of moving the outboard engines from a wingtip position to an underwing position towards the end of the wings.
On 18 September 1947, the USAAF became a separate service as the newly-established U.S. Air Force. A few months later, the XB-47 prototype flew its first flight on 17 December 1947, with test pilots Robert Robbins and Scott Osler at the controls. The aircraft flew from Boeing Field in Seattle to the Moses Lake Airfield in central Washington state, in a flight that lasted 52 minutes. There were no major problems, except that Robbins had to pull up the flaps with the emergency hydraulic system and the engine fire warning lights kept popping on, the sensor technology being very unreliable at the time. Robbins reported that the flight characteristics of the aircraft were good.
The airfoil was 11 times as wide as it was thick. This unusual thinness (dry, no fuel tanks) was believed to be necessary to attain high speed (.86 Mach), but the wing's flexibility was a concern. It could flex as much as five ft (1.5 m) up or down, and major effort was expended to ensure that flight control could be maintained as the wing moved up and down. As it turned out, most of the worries proved unfounded. (Wing "twist" limited tree-top speed to 425 knots to avoid control reversal) The wings were fitted with a set of Fowler flaps that extended well behind the wing, to enhance lift at slow speeds.
The bicycle landing gear dictated by the thin wing consisted of a pair of large wheels fore and aft of the bomb bay, with small outrigger wheels carried on the inboard twin-jet pods.
Fuel capacity was an enormous 17,000 U.S. gal (64,400 liters), compared to 5,000 U.S. gal (19,000 L) on the B-29. That meant that maintaining fuel trim to ensure a stable center of gravity in flight would be a very critical co-pilot duty.
The first prototypes were fitted with General Electric J35 turbojets, the production version of the TG-180, with 3,970 lbf (17.7 kN) of thrust. Early jet engines did not develop good thrust at low speeds, so to assist in takeoffs in heavily loaded condition, the XB-47 prototype had provisions for fitting 18 solid-fuel rocket-assisted takeoff (RATO) rockets with 1,000 lbf (4.4 kN) thrust each. Fittings for nine such units were built into each side of the rear fuselage, arranged in three rows of three bottles.
The aircraft was so aerodynamically slick that rapid descent ("penetration") from high cruise altitude to the landing pattern required dragging the deployed rear landing gear.
Unusually heavy wing loading (weight/wing area) required a high (180 knot) landing speed. To shorten the landing roll Air Force test pilot Major Guy Townsend promoted the addition of a 32 ft (9.75 m) German-designed "ribbon" drag chute. (Jet engine thrust reversers were still a far-future concept.). As a consequence, the B-47 was the first mass-produced aircraft to be equipped with anti-lock braking system.
Total bombload capacity was to be 10,000 pounds (4.5 tonnes). Production aircraft were to be equipped with state-of-the-art electronics for navigation, bombing, countermeasures, and turret fire control.
Flight testing of the prototypes was particularly careful and methodical, since the design was so new in many ways. The prototypes initially suffered from "Dutch roll", an instability that caused the aircraft to weave in widening "S" turns. This problem was remedied by the addition of a "yaw damper" control system that applied rudder automatically to damp out the weaving motion. The prototypes also had a tendency to pitch up. This problem was solved by adding small vanes called "vortex generators" onto the wings that caused turbulence to prevent airflow separation.
In early 1948, the United States Air Force (having become a separate service in 1947) sent up a chase plane from Muroc (now Edwards) Air Force Base in California to help calibrate the bomber's airspeed system. Robbins reported later:
[The chase plane] was a P-80 [Lockheed Shooting Star] and Chuck Yeager was flying it. Chuck's a hell of a good pilot, but he had a little bit of contempt for bombers and a little disdain for civilian test pilots. Well, we took off, climbed out, and got up somewhere within four or five points of full throttle speed. At that point, Chuck called me on the radio and said: "Bob, would you do a 180?" I thought: Hey, Chuck's smart, he just wants to stay reasonably close to Moses Lake, he doesn't have as much fuel as I do. Well, I turned around, got stabilized, and looked for Chuck. He wasn't there. Finally, I got on the radio and said: "Chuck, where are you?" He called back and rather sheepishly said: "I can't keep up with you, Bob." So Chuck Yeager had to admit to a civilian test pilot flying a bomber that he couldn't keep up! That was something!
Yeager would test-fly the XB-47 later in its development cycle and would years later note that the aircraft was so aerodynamically clean that he had difficulty putting it down on the runway.
It took the Air Force until 1953 to turn the B-47 into an operational aircraft. The aircraft was sluggish on takeoff and too fast on landings, a very unpleasant combination. If the pilot landed at the wrong angle, the aircraft would "porpoise", bouncing fore-and-aft. If the pilot didn't lift off for another go-round, instability would quickly cause the bomber to skid onto one wing and cartwheel. Because the wings and surfaces were flexible and bent in flight, low altitude speed restrictions were necessary to ensure effective flight control.
Improved training led to a good safety record, and few crews felt the aircraft was unsafe or too demanding, but apparently there were some aircrews who had little affection for the B-47. Crew workload was high, with only three crew members to keep the B-47 flying right. The B-52 Stratofortress, in contrast, generally had six crewmen, with more space.
Several models of the B-47 starting in 1950 included a fuel tank inerting system, in which dry ice was sublimated into carbon dioxide vapor while the fuel pumps operated or while the in-flight refueling system was in use. The carbon dioxide was then pumped into the fuel tanks and the rest of the fuel system, ensuring that the amount of oxygen in the fuel system was low, and thereby reducing the probability of an explosion. Ten carbon dioxide tanks and heaters were involved. The system was implemented largely to reduce risks from static electricity discharges occurring during in-flight refueling.
Initial mission profiles included the loft bombing of nuclear weapons. As the training for this imposes repeated high stress, the airframe lifetime would have been severely limited by metal fatigue, and this maneuver was eliminated.
Crews were also trained to perform "minimum interval takeoffs (MITO)", with one bomber following the other into the air at intervals of as little as 15 seconds, to launch all bombers as fast as possible. MITO could be hazardous, as the bombers left turbulence and, with first generation turbojet engines with water injection systems, dense black smoke that blinded pilots in the following aircraft.
B-47 bombers apparently performed training missions in which they penetrated Soviet airspace in numbers. The facts behind these missions remain controversial, with some claiming that Curtis LeMay ordered them without presidential knowledge or approval.
The B-47 would be the backbone of SAC into 1959, when the B-52 began to assume nuclear alert duties and the B-47 wings started to be cut back. B-47 production ceased in 1957, though modifications and rebuilds continued after that.
Operational practice for B-47 bomber operations during this time went from high altitude bombing to low altitude strike, which was judged more likely to penetrate Soviet defenses. Bomber crews were trained in "pop-up" attacks, coming in at low level at 425 knots and then climbing abruptly near the target before releasing a nuclear weapon, and the similar "toss bombing" procedure, in which the aircraft released the weapon while climbing, and then rolled away to depart the area before bomb detonation.
One of the more notable mishaps involving a B-47 occurred on 5 February 1958 near Savannah, Georgia. A B-47 based out of Homestead AFB, Florida was engaged in a simulated combat exercise with an F-86 Sabre, the bomber simulating an attacking aircraft and the fighter a defender. As was the practice at the time, the B-47 was carrying a single 7,600 lb Mark 15 thermonuclear weapon without its core. During this exercise, the F-86 collided with the B-47. The F-86 pilot ejected and the fighter crashed, while the B-47 suffered substantial damage, including loss of power on one of its outboard jet engines. The bomber pilot had to "safe" soft drop the Mark 15 weapon off the coast of Savannah, Georgia near Tybee Island after three unsuccessful landing attempts at Hunter Air Force Base. The bomb was successfully jettisoned and the aircraft landed safely. An extensive nine-month search was mounted for the unarmed bomb, but proved futile.
The only B-47s to see anything that resembled combat were the reconnaissance variants. They operated from almost every airfield that gave them access to the USSR, and they often probed Soviet airspace, and on occasion, B-47 pilots were caught in situations from which mostly speed and evasion in retreat saved them. At least five of these aircraft were fired on, and three of these were shot down. The B-47s fired back with their tail turrets, though it is uncertain if they scored any kills, but in any case these were the only shots fired in anger by any B-47. These missions became impractical upon the introduction by the Russians of the trans-sonic MiG-19.
Final phaseout of B-47 bomber wings began in 1963, and the last bombers were out of service by 1965. The very last USAF operational aircraft was grounded in 1969. The U.S. Navy kept specialized B-47 test aircraft in occasional use up to 1976. The final recorded flight of a B-47 was on 17 June 1986, when a B-47E was flown from the Naval Air Weapons Station China Lake, California, to Castle Air Force Base, California, for static display in the museum. There are at least 15 B-47s on static display, none flying.
The first ten aircraft were designated "B-47A", and were strictly evaluation aircraft. The first was delivered in December 1950. The configuration of the B-47As was close to that of the initial XB-47 prototypes. They were fitted with J47-GE-11 turbojets, offering the same 23 kN (5,200 lbf) thrust as the earlier J47-GE-3, and they also featured the built-in RATO bottles.
Four of the B-47As were fitted with the K-2 bombing and navigation system (BNS), with an HD-21D autopilot, an analog computer, APS-23 radar, and a Y-4 or Y-4A bombsight. Two were fitted with the tail turret, one of them using an Emerson A-2 fire control system (FCS), another an early version of the General Electric A-5 FCS. The eight other B-47As had no defensive armament.
The B-47As were fitted with ejection seats. The pilot and copilot ejected upward, while the navigator had a downward ejection seat built by Stanley Aviation. Minimum safe ejection altitude was about 500 ft (150 m).
While the XB-47s had been built by Boeing at their Seattle, Washington, plant, the B-47As and all following Boeing B-47 production were built at a government-owned factory in Wichita, Kansas, where the company had built B-29s in the past. The switch was made as the Seattle plant was burdened with KC-97 Stratotanker production and other urgent tasks.
Most of the B-47As were phased out of service by early 1952, though one did perform flight tests for NACA for a few more years. While the Air Force put the B-47As through their paces, the Cold War was rising to full force, with a hot war intensifying in Korea. The USAF's Strategic Air Command (SAC) needed an effective nuclear deterrent to keep the Soviet Union in line, and the Stratojet was an excellent tool for the task, and Boeing was already working on production bombers.
The USAF was impatient to get their hands on as many B-47s as they could as quickly as possible, and signed up Lockheed and Douglas for the additional production. Lockheed-built aircraft were designated by a "-LM (Lockheed Marietta)" suffix and Douglas-built aircraft given a "-DT (Douglas Tulsa)" suffix. Boeing production was designated by a "-BW (Boeing Wichita)" suffix, except for the Seattle-built XB-47s and B-47As, which had a "-BO" suffix.
The initial batch of 87 B-47Bs featured the same J47-GE-11 engines as the B-47As, but all subsequent production featured substantially uprated J47-GE-23 turbojets with 5,800 lbf (26 kN) thrust. Early production was retrofitted with the improved engines. They all featured the built-in RATO system used on the XB-47 and B-47A.
The K-4A used a periscopic bombsight fitted into the tip of the nose of the aircraft, with the transparent plexiglas nose cone of the XB-47 and B-47A replaced by a metal nose cone. There were four small windows on the left side of the nose and two on the right. Another visible change from the earlier models was that the B-47B had a vertical tailplane with a squared-off top, rather than a rounded top as with its predecessors.
In practice, even the enormous fuel capacity of the B-47 was still not enough to give it the range the Air Force wanted, and in fact there had been substantial prejudice against the type among senior Air Force brass because of the limited range of the initial design. Solution of this problem was a high priority, and so an "in-flight refueling (IFR)" receptacle was fitted in the right side of the nose for "boom"-style refueling. This was the main reason for getting rid of the plexiglas nose cone.
The B-47B was also fitted with a pair of jettisonable external tanks, carried between the inboard and outboard engine assemblies. These big drop tanks were very large, with a capacity of 6,750 liters (1,780 U.S. gal).
The B-47B suffered a considerable gain in weight compared to the B-47A, and so as a weight-reduction measure the ejection seats were deleted, and a windbreak panel was fitted to the aircraft's main door to make escapes easier. Some sources also claim that a fatal ejection-seat accident in a B-47A contributed to this decision. Whatever the case, this was not a very popular measure with crews, as getting out of the aircraft even at altitude was troublesome.
The designations B-47C and B-47D were applied to special variants that never went into production (described later), and so the next production version of the B-47 was the definitive B-47E.
The first B-47E flew on 30 January 1953. Four "blocks" or "phases" of the B-47E were built, each incorporating refinements on the previous block, and also sometimes featuring production changes within a block. Older blocks were generally brought up to the specifications of later blocks as they were introduced.
Early production "B-47E-Is" also known featured J47-GE-25 turbojets with 27 kN (5,970 lbf) thrust, but they were quickly changed to J47-GE-25A engines, which featured a significant improvement in the form of water-methanol injection. This was a scheme in which a water-methanol mix was dumped into the engines at takeoff, increasing mass flow and so temporarily kicking the thrust up to 32 kN (7,200 lbf). Methanol was apparently added to the water as an anti-freezing agent. The engines left a trail of black smoke behind them when water-methanol injection was on.
B-47s rarely used JATO for takeoffs, as it was expensive and slightly more hazardous than a non-assisted takeoff. Apparently it was reserved for emergency alerts, when bombers had to get off the runway as fast as possible, and was otherwise only done once a year or so as a training measure. Water-methanol injection was a big help on takeoffs when JATO wasn't used.
The internal fuel capacity of initial production B-47Es was cut to 14,627 U.S. gal (55,369 liters) as a weight-saving measure. This was considered acceptable because of the use of the big external tanks and the fact that the USAF had refined mid-air refueling to the point where it could be relied upon as a standard practice.
One welcome change in the B-47E relative to the B-47B was the return of the ejection seats. The Air Force senior leadership had reconsidered the decision to delete them and realized it didn't make sense. In addition, the twin .50 cal guns (12.7 mm) in the tail turret were replaced with twin 20 mm cannon to provide more punch, backed up by an A-5 FCS in early production and an MD-4 FCS in later production.
The B-47E-II featured only minor changes from late production B-47E-Is. The B-47E-III featured an ECM suite, consisting of a radar jammer in a bulge under the fuselage plus a chaff dispenser, as well as improved electrical alternators.
The B-47E-IV was a much more substantial update, featuring stronger landing gear, airframe reinforcement, greater fuel capacity, and a bombload uprated to 25,000 pounds (11,300 kg), though the bomb bay was once again shortened because of the introduction of more compact nuclear weapons.
Another improvement was the introduction of the MA-7A BNS, a major step up from its predecessors. The MA-7A included the AN/APS-64 radar, with a range as long as 240 miles (390 km). The AN/APS-64 could be used as a long range "identification friend or foe (IFF) transponder" interrogator to allow a B-47E-IV to find a tanker or other B-47, or it could be used as a high-resolution ground-targeting radar. The B-47E-IV retained the optical bombsight, though this was rarely used.
A total of 1,341 B-47Es were produced. 691 were built by Boeing, 386 were built by Lockheed, and 264 were built by Douglas. Most B-47Bs were rebuilt up to B-47E standards. They were given the designation of B-47B-II, though it appears that in practice they were simply called B-47Es.
The B-47E was also the basis for a number of important long-range reconnaissance variants.
Boeing-Wichita built 240 RB-47E reconnaissance variants, similar to the B-47E but with a nose stretched by 34 inches (0.86 m), giving them an arguably more elegant appearance than the bomber variants of the B-47. The long nose was used to stow up to 11 cameras, which could include:
The RB-47E could carry photoflash flares for night reconnaissance. Although the RB-47E could be refueled in flight, its fuel capacity was increased, to a total of 70,000 liters (18,400 U.S. gal). The navigator controlled the cameras, becoming a "navigator-photographer" instead of a "navigator-bombardier".
The RB-47H and ERB-47H were highly capable aircraft, but the EWO compartment was not only cramped with sitting room only, but also had both poor noise insulation and climate control. This made 12 hour missions very uncomfortable and tiring, and some sources say that the Crows even had to deal with fuel leaks on occasion. Successful ejection downward (cutting through the belly radome) was impossible on-or-near the ground. Crows sat bobsled-like on the pilot compartment access floor for takeoff and landing; having to crawl encumbered with Arctic clothing with parachute to-from their compartment along an unpressurized maintenance shelf during temporary leveloff at 10,000 ft.
Operations of the RB-47H and ERB-47H were often classified Top Secret, with the 10 hour missions generally flown at night. When crews were asked what they were doing, they always answered that such information was classified. On inquiries on what the blunt black nose was for, they would sometimes reply that it was a bumper, used in in-flight refueling in case they nosed into the tanker. This reply was often believed.
On 8 May 1954, after a top secret reconnaissance mission in the Kola Peninsula, a 4th Air Division 91 Strategic Reconnaissance Wing RB-47E reconnaissance aircraft, with Hal Austin at the controls, flew west from the Soviet Union while being chased by three Soviet MiG-17 jet fighters. The Soviet fighters had tried to destroy the RB-47E with their guns in Soviet and Finnish airspace, but the damaged RB-47E managed to escape over Sweden back to RAF Fairford station in Gloucestershire, England, where it had taken off, thanks to its remarkable top speed and combat radius superior to the Soviet fighter jets. It was the first mission in which a jet airplane equipped with modern photography equipment was used by American military reconnaissance. The incident was kept secret by all parties.
On 1 July 1960, a PVO Strany MiG-19 shot down an RB-47H (AF Serial No. 53-4281) reconnaissance aircraft in the international airspace over the Barents Sea with four of the crew killed and two captured by the Soviets but released in 1961. The co-pilot reported that the MiG-19 jammed (whited-out) his MD-4 FCS scope rendering the RB-47H defenseless.
While a few of these aircraft performed special duties during the Vietnam War, such as relaying ELINT data from drones, they were eventually replaced by much more comfortable and capable Boeing RC-135 platforms. The last RB-47H was retired on 29 December 1967.
The final 15 RB-47Es built were fitted with additional equipment, including the AN/APD "side looking airborne radar (SLAR)" system, and gear to sample the air for fallout from nuclear tests. The Air Force judged them different enough on delivery, beginning in December 1955, to give them a new designation of RB-47K.
The RB-47Ks were generally used for weather reconnaissance missions, carrying a load of eight "dropsonde" weather sensors that were released at various checkpoints along the aircraft's flight path. Data radioed back from the dropsondes was logged using equipment operated by the navigator. The RB-47Ks stayed in service until 1963.
Incidentally, there were B-47F, B-47G, and B-47J variants, but these were all one-shot conversions of B-47Bs or B-47Es, to be discussed later. There never was a B-47I variant. The Air Force never designated a B-47I, because the "I" suffix was too easily confused with the numeral "1."
Fatal Crash at Eglin - On 20 August 1963, a QB-47 veered off course on its landing approach at Eglin Air Force Base and crash landed on a stretch of road that ran parallel to the runway. Two cars were crushed by the crash landing, killing two occupants, Robert W. Glass and Dr. Robert Bundy, and injuring a third, Dorothy Phillips. Mr. Glass and Dr. Bundy both worked for the Minnesota Honeywell Corporation at the time, a firm which had just completed flight tests on an inertia guidance sub-system for the X-20 Dyna-Soar project at the base. Mrs. Phillips was the wife of Master Sergeant James Phillips, a crew chief at the base. Mrs. Phillips was treated for moderate injuries and released later that day. Both vehicles were very badly damaged. The QB-47 that crashed was used for Bomarc Missile Program tests, which normally operated from Auxiliary Field Three (Duke Field), approximately 15 miles from the main base.
B-47E conversions and special modifications included:
The total number of B-47s built was 2,032.
The B-47 undeniably fulfilled its design objectives. Perhaps as importantly, it led directly to the Boeing B-52, which:
The B-47 was succeeded by the B-58 Hustler supersonic bomber, which in turn was replaced by the FB-111. The XB-52 used the fighter style canopy, and tandem landing gear, but subsequent large bombers have used side-by-side flight decks.
The 1 July 1960 shoot down of an RB-47H (s/n 53-4281) by a MiG-19 over the Barents Sea was retold by the two surviving crewmembers, Capt. John R. McKone (navigator) and Capt. Freeman B. Olmstead (co-pilot), in the biographical book The Little Toy Dog by William L. White. The title refers to a small plastic toy "Snoopy" that Capt. McKone carried with him, and kept during their seven months in Lubyanka prison in the Soviet Union.