The X-15 was designed to explore hypersonic speeds in excess of Mach 6 and altitudes over 300,000 feet. The aircraft was powered by a rocket engine dubbed "the million horsepower engine". There were three X-15 aircraft built and they were flown from 1959 though 1968.
The number two craft was damaged in a crash. It was modified with the fuselage being stretched (2.4 ft) and the addition of two external drop tanks. This enabled the craft to carry 76% more propellant. The new craft was designated X-15A-2. During one of the flights, with a special ablative coating, it was able to attain the unofficial record speed of Mach 6.7 (4,520 MPH) in October 1967 with pilot William "Pete" Knight in control.
This is my depiction of the X-15A-2 in flight on November 3, 1965. It was the first flight with the external tanks. The tanks were empty. The craft survives today at the National Museum of the United States Air Force.