The USAF eventually gave up fighting for its production and the B-70 program was canceled in 1961. Development was then turned over to a research program to study the effects of long-duration high-speed flight. As such, two prototype aircraft, designated "XB-70A", were built; these aircraft were used for supersonic test-flights during 1964–69. In 1966, one prototype crashed after colliding with a smaller aircraft while flying in close formation; the remaining Valkyrie bomber is in the National Museum of the United States Air Force near Dayton, Ohio.