Boeing XB-39 Superfortress

The Boeing XB-39 Superfortress was a United States prototype bomber, a single B-29 Superfortress converted to fly with alternative engines. It was meant to show that the B-29 could still enter service if its first-choice air-cooled Wright R-3350 radial ran into development or production trouble. It began life as the first YB-29 delivered to the Army Air Forces and went in November 1943 to the Fisher Body Aircraft Development Section of General Motors, which converted it to Allison V-3420-17 liquid-cooled W24 inline engines, since Fisher knew the engine from its P-75 Eagle. Delays in the planned turbosuperchargers meant the first flights had to be made without any. The first flight was on 9 December 1944 at Cleveland, Ohio, and early tests were impressive, but the programme was by then badly delayed and the B-29 was already in combat, so it was not ordered into production.