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.

Despite its technical triumphs and support from aviators, the XB-70’s operational future unraveled almost as quickly as it took flight. The North American XB-70 Valkyrie emerged from a United ...