Boeing Model 322 Superfortress predecessor “What If”
The jump between Boeing’s two WWII bombers, the B-17 and B-29, might seem like a quantum leap, as the B-17 appeared near the beginning of the monoplane era, while the B-29 was a pressurized monster of an aircraft that could carry nuclear weapons. Yet there was a design lineage between the two, and there were a number of unbuilt intermediate designs. An early example of these was the Boeing Model 322, which was an attempt to adapt early B-17 wings and tail to a pressurized fuselage – in essence a bomber counterpart to the Model 307 airliner.
American Secret Projects: Fighters, Bombers, and Attack Aircraft, 1937-1945; Chapter Four: USAAF Heavy Bombers has details on the Model 322 and other intermediate designs such as the Models 333, 334A and 341.