It actually is not the fastest plane. But the fastest plane did not make the circuit that was required for the official speed record, since the X-15 flew in more or less a straight line.
The North American X-15, a rocket-powered research aircraft, bridged the gap between manned flight in the atmosphere and space flight. After its initial test flights in 1959, the X-15 became the first winged aircraft to attain hypersonic velocities of Mach 4, 5, and 6 (four to six times the speed of sound) and to operate at altitudes well above 30,500 meters (100,000 feet).
It doesn't answer your question, but does set the story straight.