The North American Aviation XB-70 Valkyrie prototype was planned as a nuclear-armed, deep-penetration strategic bomber for the United States Air Force Strategic Air Command. The six-engined Valkyrie was capable of cruising thousands of miles at Mach 3+ while flying at 70,000 feet. Two aircraft were built but with the advent of ICBMs, manned bombers were seen as obsolete and it did not go into production.