By the time World War II started in earnest, electrically-heated suits were introduced for patrol and bomber crews who routinely operated at high altitudes above 30,000 feet, where air temperatures could get so cold that flesh could freeze instantly to any metal in which it came in contact. As pressurized cabins came into operation, the necessity of wearing bulky leather and shearling jackets and pants began to fade. For example, pilots, navigators, and bombardiers of a B-17 operating in Europe in 1944 would wear their officer's uniforms under an A-2 flight jacket comfortably due to the enclosed and heated cabin, but the waist gunners would have to wear electrically-heated suits as they fired their guns through open window gunports. When the Superfortress was introduced in the fight against Japan, along with remote-controlled coordinated gun turrets, the fully-pressurized crew cabin made the necessity of bulky flight gear obsolete.
Where bomber pilots could get away with wearing their dress uniforms as flight gear, fighter pilots needed a uniform that functioned in the tight confines of the typical fighter plane cockpit. The AN-S-31 flight suit was developed for the US Army Air Corps and featured two button-down breast pockets and two button-down shin pockets that could be accessed from the sitting position. The US Navy used a slightly different model that featured slanted pockets with zippers. The material used was either wool or tight-weave cotton for wind resistance and fire protection.
The need for short-duration fire protection was demonstrated early during that war. As technology advanced, the fire-protective flight suit, helmets, goggles, masks, gloves and footwear were designed and used. The footwear often could be cut to appear like civilian shoes in the country where the crew member would land if shot down.
Flak jackets were also developed to give bomber crews some protection from flying shrapnel, though these increased the overall weight of the airplane and reduced the effective bombload that could be carried.
With the era of jet flight and improved focus on safety, however, fully fire-retardant materials were required. It was also simpler to make a one-piece suit when it would potentially have to fit over existing clothing or various types of under-garments.
Also, with the coming of jet flight came the development, in Canada, of the G-suit, a special kind of flight suit (worn alone or in combination with a traditional flight suit) that protected the wearer from the physical stress of acceleration by compressing the body to keep blood from pooling in the legs. As the pilot executed high-G combat maneuvers, his blood would literally be pulled from his head and shift downwards into his lower body, starving the brain of oxygen and causing a blackout. The G-suit was designed to allow some retention of blood in the pilot's head, allowing him to execute high-G turns for sustained periods of time.
In the 1950s and 1960s even more specialized suits needed to be developed for high-altitude survelliance (such as with the U-2 and SR-71 aircraft) and space flight. These would include full pressurization, and would be the precursor to today's space suits.