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The Boeing YB-52 was the second prototype aircraft built and was virtually identical to the XB-52. The YB-52 was initially ordered as the second XB-52, but various changes incorporated into the aircraft on the assembly line warranted a designation change.
The first prototype was given the designation "XB-52" and the second the designation "YB-52". The second prototype was given a "Y" code, which would normally indicate an evaluation machine, not an "X" code as was appropriate to its experimental status, because the Air Force had scrounged funding for it from their Logistics Command, which was not formally allowed to fund experimental aircraft.