The XB-70 Valkyrie made its maiden flight in September 1964.
The first XB-70 Valkyrie built, the XB-70A, had handling at speeds greater than Mach 2.5, and only got above Mach 3 once. So a second one was built, the XB-70B, with the wings adjusted just 5 degrees, which helped handling. The XB-70B flew for the first time in July 1965.
Source: NASA
It had six General Electric J-93 turbojets, each providing the XB-70 with 30,000 pounds of thrust.
Source: Boeing
On June 8, 1966, while conducting high-speed flight research, the XB-70 collided with NASA's F-104N chase plane, killing the F-104 pilot and the XB-70 co-pilot. The other XB-70 pilot was able to eject and survived, albeit with serious injuries.
After the destruction of the XB-70B, only the first XB-70A remained.
Source: NASA
The XB-70 continued flying for research purposes until February 1969, and much of the research done contributed to the development of the B-1 bomber.
Source: Popular Mechanics