Boeing's Model 307 Stratoliner was the first four-engined transport with a pressurized cabin. It appeared prewar in two models: Model 307, built for Pan American; Model 307B, built for TWA. The differences were minor, as both types were conscripted for Air Transport Command use as C-75s.

The plane was designed and built in parallel with the B-17 and many components were shared with the B-17B. The demands of pressurization and passenger comfort dictated the cigar-shaped circular-cross-section fuselage. After considerable wartime service, TWA planes were refurbished as Model 307B-1 types, using wing, fin, stabilizer and other components from the B-17G.