The XB-39 "Spirit of Lincoln" was a modification of the first YB-29 (S/N 41-36954). The XB-39 project was similar to the XB-38 project -- the standard air-cooled radial engines of the production aircraft were replaced by liquid cooled 'Vee' engines. On the XB-39, the original Wright R-3350-21 Cyclone radials were replaced by Allison V-3420-11 engines. The V-3420 was essentially two V-1710 engine spliced together (one crankcase and two crankshafts geared together).
The XB-39 project was basically a proof-of-concept project to demonstrate performance with liquid-cooled "Vee" engines. It was also insurance against shortages of the production engine. Only one XB-39 was built; it was delivered the U.S. Army Air Force in early 1944 for testing. Most of the problems with the standard B-29 production version were fixed by mid-1944, and no orders for the XB-39 were placed.
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)