The XB-70 Valkyrie, a Mach 3-capable bomber developed during the Cold War, was designed to outpace Soviet air defenses. Powered by six turbojet engines, it could reach 2,076 mph at 73,000 feet.

erasing much of the XB-52’s speed advantage. But, the Air Force planners said, if Boeing could come up with an all-jet, long-range bomber.... That was on a Friday. Boeing’s team, led by ...