This was the second of three X-15 a/c. After a mishap, it was rebuilt into an improved version. This included external propellant tanks for longer burn time and a slightly longer fuselage. In October 1967, being flown by Pete Knight, this X-15 set a speed record of Mach 6.7 (4,520 miles per hour) which still stands today. Transferred to the museum in 1969 after the X-15 program ended.