Boeing YB-29-BO (S/N 41-36957). Note the nose has an Erco ball turret with twin .50-cal. machine guns, and the fuselage package guns just below and aft of the cockpit have twin .50-cal. machine guns mounted. (U.S. Air Force photo)
The YB-29 was an improved version of the XB-29 used for service testing. The engines were upgraded to the -21 model of the Wright Cyclone, and three-blade propellers of the XB-29 were changed to a four-blade type for the YB-29. The YB-29 featured an improved fire control system and turret-mounted .50-cal. machine gun pairs.
The first flight of the YB-29 took place in the summer of 1943, and as testing proceeded, a wide variety of modifications were tried on one or more of the 14 aircraft ordered. The first YB-29 (S/N 41-36954) was modified by replacing the Wright Cyclone radial engines with Allison V-3420-17 inline engines and was redesignated XB-39. The fourth YB-29 (S/N 41-36957) was extensively modified to test defensive armament. One arrangement had a nose-mounted Erco ball turret and fixed forward fuselage blisters housing dual .50-cal. machine guns. Other YB-29s were fitted with manned dorsal and ball turrets (similar to the B-17 arrangement), waist gunner positions and remote-controlled ball turrets mounted in the cheeks (lower forward nose).
Because of the urgent need for the long-range B-29, production aircraft were developed and built in parallel with the service test program. The first of more than 1,600 B-29-BWs built at Boeing's Wichita, Kan., plant was delivered in September -- just two months after the initial YB-29 was delivered.
Improved XB-29 for service test
TECHNICAL NOTES: Armament: 10 .50-cal. machine guns (standard) plus 20,000 lbs. of bombs Engines: Four Wright R-3350-21 Cyclone radials of 2,200 hp each (takeoff power) Maximum speed: 368 mph at 25,000 ft. Cruising speed: 247 mph Range: 5,850 miles Service ceiling: 32,100 ft. Span: 141 ft. 3 in. Length: 98 ft. 2 in. Height: 27 ft. 9 in. Weight: 120,000 lbs. (gross) Crew: 11 Serial numbers: 41-36954 to 41-36967