The North American XB-70 Valkyrie was the largest and fastest bomber ever built by the United States, but the massive six-engine Mach 3.0-capable jet never entered production. Only one surviving prototype sits in a museum in Dayton, Ohio , even as the Boeing B-52 it was supposed to one day replace continues to soldier on.
The idea behind the XB-70 originated in the 1950s when it was assumed ever-greater speeds and altitudes would enable American bombers to survive against Soviet air defenses unmolested on their way to delivering their doomsday payloads. At the time, the only effective defense against bombers were fighters and antiaircraft artillery. Even then, anti-aircraft guns were only marginally effective and interceptors were increasingly challenged by ever improving bomber performance.
However, with the advent of surface-to-air missiles (SAM), that began to change—the balance started to tip in favor of the defender. While the U.S. Air Force was aware of Soviet advances in SAM technology, the Pentagon didn’t start to understand the scope of the problem until Francis Gary Powers’ Lockheed U-2 spy plane was shot down while overflying the Soviet Union on May 1, 1960. But development of the XB-70 continued nonetheless.
With the growing realization that Soviet SAMs posed an increasing threat to American bombers, the Pentagon started to explore low-level penetration as an alternative. Low-level penetration involved flying under the radar horizon using terrain to mask a bomber’s approach, which greatly reduces enemy response times. Moreover, the development of intercontinental ballistic missiles greatly reduced the United States’ reliance on manned bombers. Many leading military strategists of the time believed bombers were too vulnerable to survive  the journey into Soviet airspace. As a result, President John F. Kennedy decided to cancel the XB-70 as a frontline bomber program on March 28, 1961.
Meanwhile, the XB-70 test program continued. The jet made its first flight on Sept 21, 1964, when it flew from Palmdale to Edwards Air Force Base, Calif. But the first XB-70 proved to be a disappointment—it had poor directional stability above Mach 2.5 and made only one flight above Mach 3.0. The second jet, which flew on July 17, 1965, added five degrees of dihedral on the wings for better supersonic stability.
Tragedy struck on June 8, 1966, when the second XB-70 prototype was destroyed in a crash after a midair collision with its F-104N chase plane. Two people were killed and one was severely injured during the accident. The loss of the second aircraft—which was much more capable than the first—was a huge set back. Testing, however, continued until Feb. 4, 1969. Ultimately, the first XB-70 logged eighty-three flights totaling 160 hours and sixteen minutes, while the second XB-70 logged forty-six flights totaling ninety-two hours and twenty-two minutes according to NASA.
The XB-70, while a technological wonder at the time, was the wrong plane for the wrong time. It came at a time when ballistic missiles were thought to be supplanting manned bombers. Moreover, it was being developed at a time when it was increasingly apparent that high speed and high altitude were not sufficient protection against surface-to-air missiles or the next generation of Soviet fighters.
But the nail in the coffin was the jet’s exorbitant price tag and lack of mission flexibility—the B-70 couldn’t be adapted for the low level role. Let’s hope today’s shadowy Long Range Strike Bomber fares better .
Dave Majumdar is the defense editor for The National Interest. You can follow him on Twitter: @davemajumdar .