Boeing XB-39 Superfortress - Design and Development

Design and Development

Starting life as the first YB-29 delivered to the United States Army Air Forces, it was sent in November 1943 to the Fisher Body Aircraft Development Section of General Motors to be converted to use Allison V-3420-17 liquid-cooled X24 (twin-V12, common crankcase) inline engines. Fisher was chosen for the modification as it was familiar with the engine, as it was to power the P-75 Eagle that they were then developing. Testing on it began in early 1944.

Further development of the engine and the aircraft was delayed by a series of changes in the planned turbosuperchargers as the originally specified GE Type CM-2 two-stage turbosupercharger became unavailable due to demands on GE's production of its other turbosuperchargers. Other turbosuperchargers were considered, but the end result was that the first flights of the B-39 had to be made without any turbosuperchargers at all.

In addition, in early 1944, due to a sudden realization from the U.S. Army Air Force that it required a long range air superiority fighter, Fisher was directed to focus on its other major project, the P-75 Eagle. In June 1944, Fisher received a contract for 2,500 P-75s. However, in October 1944, for a variety of reasons, the P-75 was canceled.