Boeing XB-39

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.

Sources:

1. Warbird History--B-29 Superfortress, Chester Marshall, Motorbooks International, 1993.
   
   
2. The Boeing B-29 Superfortress, Mich Mayborn, Aircraft in Profile, Doubleday, 1969.
   
   
3. B-29 Superfortress, John Pimlott, Gallery Books, 1980.
   
   
4. Famous Bombers of the Second World War, William Green, Doubleday, 1960.
   
   
5. Boeing Aircraft Since 1916, Peter M. Bowers, Naval Institute Press, 1989.
   
   
6. United States Military Aircraft Since 1909, Gordon Swanborough and Peter M. Bowers, Smithsonian Institution Press, 1989.
   
   
7. Jane's Fighting Aircraft of World War II, Bill Gunston, Military Press, 1989.
   
   
8. American Combat Planes, Third Enlarged Edition, Ray Wagner, Doubleday, 1982.