In early 1934 Boeing began design studies for a multi-engine bomber and a basically similar civil transport. When, in June 1934, the USAAC invited proposals for a new bomber, Boeing's Model 299 was revamped to meet the Army specification and duly became built by the thousands as the B-17 Flying Fortress.

The Model 300 was also changed as ideas were developed and in its Model 307 form was basically similar to the B-17, except for a very different fuselage. This was of circular cross-section, a configuration chosen so that the structure would be able to withstand the stresses of pressurisation, thus ensuring a smooth flight for the crew of five and 33 passengers at, an altitude above much of the atmospheric turbulence.

The prototype Model 307 (named Stratoliner because of its high cruising ceiling) flew for the first time on 31 December 1938. A total often were built: the prototype; five SA-307B for Transcontinental & Western Air; three S-3O7 for Pan American; and a single SB-307B for the late Howard Hughes. TWA's SA-307B were impressed for service with the Army's Air Transport Command during World War II, being used as C-75 for VIP transport over the North and South Atlantic.

The Model 307 had the distinction of being the first civil airliner with a pressurised cabin to enter service. It also introduced an extra crew member - known as the flight engineer - to relieve the captain of certain duties such as power plant, fuel and pressurisation management and monitoring