When the B-52 Stratofortress entered operational service with the US Air Force in 1955, work was already underway on defining its successor. The B-70 Valkyrie, a Mach 3 jet bomber, was one option. Although two XB-70A prototypes flew, the B-70 never went into production. Out of the subsequent Advanced Manned Strategic Aircraft program came the B-1A bomber, which flew at high speed and low altitude to evade enemy air defenses. The B-1A was cancelled in favour of fitting the B-52 with cruise missiles. The B-1, known as the BONE, was revived in 1981 as the improved B-1B to boost American...