Pressurized Cabins
While the concept seems a bit strange today, given the dangers of hypoxia, altitude sickness, decompression sickness and barotrauma while flying at higher altitudes, planes during the early part of WW2 did not have pressurized cabins. This meant that pilots were forced to rely on oxygen masks in order to get the oxygen they needed to stay conscious and fly the plane safely. These were impractical on many levels; they were bulky and restricted movement. The pilot and passengers had to stay in place in order to stay connected with the bulky oxygen mask. Even worse was that the masks were prone to failure which was exceedingly dangerous.
But the biggest catalyst for moving away from the mask was the development of larger bombers. For these bombers to function, the crew needed to be able to move about the cabin, which they could not do with oxygen masks. Prior to this pressurized cabins had existed but they had only been on experimental planes. In 1938 the Boeing 307 was a commercial plane that was developed with a pressurized cabin but only 10 of those planes were built. Then came the Boeing B-29 Superfortress. The aptly named plane was the largest aircraft in operation during World War 2 and was first put into service in 1944. The plane featured a pressurized cabin for the crew areas which allowed them to crawl back to the bomb bays without needing an oxygen mask and without having to keep the bomb bays pressurized (and therefore not have to depressurize after dropping their load).
The Boeing B-29 Superfortress was so advanced that it remained in service after World War 2 and was even used during the Korean War. After the success of the B-29 Superfortress the race to create commercial jet airliner was on. By the late 1950s, both Britain and the U.S. had successfully created commercial passenger airplanes.