The first YB-29 (41-36954) was turned over to General Motors for
installation of liquid-cooled Allison V-3420 engines and further
tests. The converted aircraft was later redesignated XB-39. The
V-3420 engine was essentially a pair of Allison V-1710 twelve-cylinder
liquid-cooled Vee engines coupled to a single propeller shaft. Normal
output was 2100 hp at 25,000 feet. Aircraft speed increased to 405
mph at 35,000 feet, but the improvement in performance was not
considered sufficient to justify production.
Warbird History--B-29 Superfortress, Chester Marshall, Motorbooks
The Boeing B-29 Superfortress, Mich Mayborn, Aircraft in Profile,
B-29 Superfortress, John Pimlott, Gallery Books, 1980.
Famous Bombers of the Second World War, William Green, Doubleday,
Boeing Aircraft Since 1916, Peter M. Bowers, Naval Institute Press,
United States Military Aircraft Since 1909, Gordon Swanborough and
Peter M. Bowers, Smithsonian Institution Press, 1989.
Jane's Fighting Aircraft of World War II, Bill Gunston, Military
American Combat Planes, Third Enlarged Edition, Ray Wagner,