 The program ended around 1964, when fatigue problems with the wing spars forced the retirement of the surviving aircraft, which was returned to the US.They were replaced by four Lockheed U-2s, all of which were subsequently lost in operations over the Chinese mainland.The pace of missions put a strain on the delicate wings of the RB-57D and the first aircraft were put into storage by SAC following two incidents when the wings outboard of the engine nacelles cracked and fell off during landing.