Designed in the late 1950s, the North American XB-70 Valkyrie prototype of the planned B-70 nuclear-armed, deep-penetration Strategic Bomber for the United States Air Force and Strategic Air Command. The $750,000,000 six-engined Valkyrie was capable of cruising for thousands of miles at Mach 3+ while flying at 70,000 feet. Fast enough to outrun any intercepters that try and get their sights on her. The only problem was...keeping paint on her surface.
The massive XB-70 had an impressive 534,700 lb loaded weight, and a robust 28,800 pounds of thrust with afterburners from her 6 GE YJ-93-GE-3 engines that put out a total of over 172 thousand pounds of thrust to help push this massive bird at over 2,020 mph, or over three times the speed of sound(Mach 3) at altitudes over 74,000 ft with a long reaching range of over 3,725 nautical miles.
At those exceptional speeds and altitudes, it was expected that the production B-70 bomber would be almost immune to interceptor aircraft, the only effective weapon against bombers at the time. The XB-70's performance could not be matched by any contemporary Soviet fighter.
The introduction of the first Soviet Surface To Air Missiles(SAM) in the late 1950s put the operational safety of the B-70 in doubt. The Valkyrie was no longer invulnerable to enemy defences. In response, the United States Air Force began flying the XB-70 missions at low level, where the massive bomber would be protected by terrain while limiting the missiles tracking to "line of sight". In this low-level penetration role, the B-70 offered little additional performance over the already operational and proven B-52 bombers it was meant to replace, while being far more expensive with shorter range. With the advent of ICBMs during the late 1950s, and their must shorter payload delivery time, manned bombers were increasingly seen as obsolete.
The USAF eventually gave up on the production of the B-70, and the program was ultimately canceled in 1961. Further development was then turned over to a research program to study the effects of long-duration high-speed flight. The two prototype aircraft were used for supersonic test-flights during 1964–69. Sadly, XB-70 No 2 was lost in a horrific mid air collision on June 1966.
Read about it here: https://sierrahotel.net/blogs/news/crash-of-the-xb-70
Today, only one of the great XB-70's still remains, and is currently on display at the National Museum of the Air Force near Dayton, Ohio.