North American XB-70 Valkyrie. The futuristic XB-70A was originally conceived in the 1950s as a high altitude, nuclear strike bomber that could fly at Mach 3 (three times the speed of sound) -- any potential enemy would have been unable to defend against such a bomber. By the early 1960s, however, new Surface-to-Air Missiles (SAMs) threatened the survivability of high-speed, high altitude bombers. Less costly, nuclear armed ICBMs (Intercontinental Ballistic Missiles) were also entering service. As a result, in 1961, the expensive B-70 bomber program was canceled before any Valkyries had been completed or flown.