In October 1964, a six-engine white delta-winged aircraft the length of a Boeing 737 rolled out of North American Aviation's Palmdale facility. The XB-70 Valkyrie was the most ambitious bomber ever built: a 500,000-pound nuclear strike platform designed to cruise at Mach 3.1 at 70,000 feet — fast and high enough that no Soviet interceptor could catch it. It flew. It worked. And then a chase plane crashed into it during a publicity photograph and the program died.

The technology was genuinely revolutionary. The XB-70 used a principle called compression lift, discovered by NASA's Alfred Eggers in 1956: at hypersonic speeds, the shock wave from the aircraft's nose could be trapped under the wing, generating up to 30% of total lift for free. Hinged wingtips folded down 65 degrees in cruise to capture this shock and provide directional stability. The airframe was built from brazed stainless steel honeycomb sandwich panels — a manufacturing process so difficult that North American had to invent new welding techniques and a vacuum-furnace brazing process from scratch. The fuel was a special high-density JP-6 that doubled as a heat sink for the airframe, which reached 330°F in cruise.

Why it died: Three forces converged in 1961–66. First, the U-2 shootdown of Gary Powers in May 1960 proved that high altitude was no longer sanctuary; the Soviet S-75 SAM could reach 80,000 feet. Second, Defense Secretary Robert McNamara argued that ballistic missiles made manned bombers obsolete and slashed the program from 250 production aircraft to two prototypes in March 1961. Third, on June 8, 1966, the second XB-70 was performing a formation photo flight for General Electric (whose engines powered all five aircraft in frame) when an F-104 chase plane piloted by NASA's Joe Walker drifted into the Valkyrie's wingtip vortex, flipped over the bomber, and sheared off both vertical stabilizers. Both aircraft crashed. Walker and XB-70 co-pilot Carl Cross died. The surviving prototype made 33 more research flights and was retired in 1969.

Why it's viable now. Every constraint that killed the Valkyrie has inverted:

• Compression lift is no longer exotic. The Boeing X-51 Waverider (2010–13) flew at Mach 5+ using the same principle. CFD modeling has replaced the wind tunnel guesswork that consumed 60% of the XB-70's $1.5B development cost (≈$15B today).
• Materials have caught up. Modern titanium-aluminide and ceramic-matrix composites tolerate 1,500°F continuously — the XB-70 was thermally limited to Mach 3. A clean-sheet design could cruise at Mach 4+ without exotic fuels.
• The mission came back. China's HQ-9 and Russia's S-500 have pushed standoff cruise missiles to their useful range limits. A penetrating high-supersonic platform — exactly what the Air Force's Next Generation Air Dominance bomber adjunct is now studying — solves the same kill chain problem the Valkyrie was designed for.
• Fly-by-wire makes the canard-delta configuration safe. The XB-70 was statically unstable in pitch at high speed and required constant pilot trim; an F-35-class flight computer makes that trivial.

The XB-70's two prototypes accumulated 129 flight hours. The surviving aircraft sits in the National Museum of the U.S. Air Force in Dayton, still the largest, fastest, highest-flying bomber ever built. Sixty years after first flight, nothing has matched it.