Boeing XB-39 Bomber Prototype

The Boeing XB-39 was a development of the Boeing B-29 Superfortress, designed to prove that the B-29 could be powered by alternate engines if the preferred option was unavailable. Allison V-3420-17 liquid-cooled W24 (twin-V12, common crankcase) inline engines were used to replace the Wright R-3350 radial engine.

Delays in the availability of the turbosupercharger meant that the first flight was not made until 9 December 1944. By this stage the B-29 was in operation and despite issues with the Wright R-3350 engines, it was performing well in combat. Rather than disrupt production with a change of engine, the XB-39 project was cancelled.