During the Battle of Britain, the Luftwaffe used Messerschmitt Bf 109s and Bf 110s based in France as escort fighter-bombers. Flying with the Heinkel He 111 and Junkers Ju 88 medium bombers, they would drop their light bomb-load and—for a brief period—fight off the British fighters. The Bf 109, however, was operating at the very edge of its range, and the Bf 110 had inferior performance; as escorts they provided only limited protection.
The plan was quickly undermined. Hitting a fast-moving fighter with guns in a turret proved extremely difficult; hitting a slow-moving bomber from a fighter with a gunsight was much easier. USAAF bomber losses gradually increased, and experimental "gunships" like the YB-40 did nothing to reduce them.
It was not until the introduction of the P-38 Lightning and P-47 Thunderbolt that the bombing raids could claim a measure of success. Able to carry large Lockheed-designed drop tanks, the fighters were able to escort the bombers for much of their missions. The first Allied fighters over Berlin were 55FG P-38s on March 3 1944. When the Merlin-powered P-51 Mustang was introduced, with a laminar-flow wing for efficiency, the final escort fighter development of the war was complete. The combination of strategic bombing and masses of powerful escort fighters allowed the USAAF to open a "second front in the air" against Germany long before a ground campaign was possible.
The XB-70 Valkyrie, North American Aviation's Mach 3 bomber, was intended to be immune to enemy attack as a result of speed. When the Soviets development of the Mach 3 MiG-25 interceptor became known, however, NAA began work on the XF-108 Rapier. The Rapier was intended to protect the XB-70, using many of the same technologies; when the bomber was cancelled, so too was the XF-108.
Still, during the Vietnam War a variation on the escort fighter was developed to suppress enemy missile defenses. The SEAD (suppression of enemy air defenses) role was given to F-100 Super Sabre and later F-105 Thunderchief fighters, nicknamed Wild Weasels, which would strike during or just before a raid.