Boeing B-29 Superfortress B-29D

The B-29D is the variant that solved the Superfortress's chronic problem and, paradoxically, never flew under its own name. From 1944 the Army Air Forces wanted to replace the unreliable Wright R-3350 Duplex-Cyclone with the Pratt & Whitney R-4360 Wasp Major, a four-row, 28-cylinder radial that was the largest-displacement piston aero engine ever built in quantity in the United States. To test the installation, Pratt & Whitney received a B-29A-5-BN, 42-93845, and fitted four 3,000 hp (2,200 kW) R-4360-33s in place of the 2,200 hp (1,600 kW) R-3350s. The aircraft, designated XB-44 Superfortress, first flew in May 1945 and proved 50–60 mph (80–100 km/h) faster than a standard B-29, although the sources do not allow the gain to be split between engine power and the weight saved by deleted armament. In the Superfortress's own history the B-29D/B-50 programme is cited as the definitive cure for the engine problems, one that arrived too late for the war.

The B-29D went far beyond an engine change. Its four 3,500 hp (2,600 kW) R-4360-35s gave nearly 60% more power than the usual Duplex-Cyclone, and the structure was redesigned to exploit it. The wing was built in 75-S aluminium alloy instead of 24ST, making it both stronger and lighter; the undercarriage was strengthened to operate at weights up to 40,000 lb (18,000 kg) greater than the B-29's; the fin and rudder grew to cope with the power (the fin folding so the aircraft would fit into existing hangars) and the flaps were enlarged for the weight. Armament was similar to the B-29's: two bomb bays carrying 20,000 lb (9,100 kg) of bombs and a further 8,000 lb (3,600 kg) externally, with 13 .50-calibre machine guns (or twelve and a 20 mm cannon) in five turrets.

An order for 200 B-29Ds was placed in July 1945, but the end of the war in August brought mass cancellations: over 5,000 B-29s were cancelled in September 1945, and in December the B-29D order was cut to 60 aircraft. At that same moment the designation was changed to B-50. Officially the new name reflected the scale of the changes, but according to Peter M. Bowers, a long-serving Boeing engineer and authority on the company's aircraft, the redesignation was a ruse to win appropriations for an aircraft that, as the B-29D, looked like just another version of a model being cancelled wholesale and sent into storage. There was no B-50 prototype: engines and tail had already been proven on the XB-44, and the first production B-50A flew directly on 25 June 1947, followed by 78 more. The production B-50A carried four R-4360-35s and a maximum take-off weight of 168,500 lb (76,400 kg).

Under the B-50 name, the re-engined Superfortress served in the American bomber fleet through the 1950s. The first batch was followed by 45 B-50Bs with lightweight fuel tanks and higher operating weights and by 222 B-50Ds, able to carry underwing tanks and recognisable by their one-piece plastic nose dome. To reach the Soviet Union they were fitted for in-flight refuelling: most B-50As with Flight Refuelling's looped-hose system, and most B-50Ds with the receptacle for the flying boom that Boeing designed for SAC. The B-50 was also the first aircraft to fly around the world non-stop, Lucky Lady II completing a 94-hour flight in 1949. The turbo-compound YB-50C derivative and its production version, the B-54, of which 43 were ordered in 1948, were cancelled in April 1949 by General LeMay, who judged them inferior to the B-36. Under the Saddletree nuclear-capability programmes 36 B-50As and 23 B-50Bs were modified; each of the 36 B-50s handled at Sacramento required 6,000 hours of work.

The B-29D was the bridge between the wartime Superfortress and that of the Cold War. With the B-36 in service the B-29 became a medium bomber and the B-50 inherited the auxiliary roles (air-sea rescue, electronic intelligence, air refuelling and weather reconnaissance); the B-47 replaced it as a bomber in the early 1950s and the last KB-50s and WB-50s were retired in 1965. What the B-29D left, above all, was the lesson that the Superfortress airframe was far better than the engines with which it went to war.