The XB-39 "Spirit of Lincoln" was a modification of the first YB-29 (S/N 41-36954).

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 W24 (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.

Fisher finally focused again on the B-39. The first flight of the B-39 was made on 9 December 1944 at Cleveland, Ohio. The initial flight tests of the B-39, without turbosuperchargers installed, were impressive. However, the B-39 program was by now seriously delayed, and the flawed R-3350 B-29s had already been rushed into combat in June 1944.

Despite continuing problems with the B-29s, the aircraft was functioning well enough in combat that it no longer made any sense to shift resources in the manufacturing base to a new engine for the B-29 and so the B-39 was not ordered into production.

TECHNICAL NOTES:
Armament: 10 .50-cal. machine guns plus 20,000 lbs. of bombs (maximum overload).
Engines: Four Allison V-3420-11 engines of 2,100 hp each (3,000 hp each, takeoff power)
Maximum speed: 405 mph at 25,000 ft.
Cruising speed: 282 mph
Range: 2,850 with 4,000 lbs. of bombs; 6,290 miles maximum ferry range
Service ceiling: 35,000 ft.
Span: 141 ft. 3 in.
Length: 99 ft. 0 in.
Height: 29 ft. 7 in.
Weight: 135,000 lbs. (maximum gross weight)
Crew: Six normal (10 with gunners; 12 with relief crew)
Serial number: 41-36954 (YB-29-BO)







Regards Duggy