The Air Force issued requirement for a new strategic bomber in November 1945. Boeing won the contract bid June 5, 1946. The B-52 design evolved from a straight-wing aircraft powered by six turboprop engines to the final prototype B-52s with eight turbojet engines.
By early 1949, Boeing was preparing two prototypes, the XB-52 and YB-52, both of which had been contracted for in early 1947. Although the YB was designated for service test, both models were used to refine the original design. These aircraft, weighing 390,000 pounds apiece, would be two of the largest aircraft ever built.
Even as these airships were being built, SAC requested Boeing to examine the possibility of developing a reconnaissance version. This was the first hint of interest in expanding the original nuclear strike mission specified for the B-52. The result of the SAC request was a design which could be assigned either a bombing or reconnaissance mission with no sacrifice in efficiency or performance. Although this capability was not integrated into the aircraft until introduction of the B-52B, it marked the initial swing towards mission flexibility in the B-52.
The major design emphasis was placed on superior performance with minimum airplane and system complexity. This was to be achieved by a straightforward design which provided a high standard of systems utility and functional reliability. To do this, however, required a number of innovations not anticipated in the original specifications. Some of the design features, like the on-board hydraulic system, were physically the largest yet built, while others like the pneumatic system which powered many aircraft accessories were radical departures from conventional designs. Nonetheless, the original flight test in 1952 proved successful, with performance exceeding the original specifications.
Two prototypes were constructed. The YB-52, the second prototype, first flew April 15, 1952. The XB-52 was damaged during a full-pressure test of its pneumatic system and did not make its first flight until Oct. 2, 1952. Built to carry nuclear weapons for Cold War deterrence, the B-52 Stratofortress replaced the Convair B-36. In total, 744 B-52s were built, with the last delivered in October 1962.