White was designated the Air Force's primary pilot for the North American X-15 program in 1958. While the new plane was undergoing its initial tests, he attended the Air Command and Staff College at Maxwell Air Force Base, Alabama, graduating in 1959. He made his first test flight of the X-15 on April 15, 1960, when the aircraft was fitted with two interim, 16,000 lbf (71 kN) XLR-11 thrust rocket engines. Four months later, he flew to an altitude of 136,000 feet (41.5 km) above Rogers Dry Lake. White would have participated in the Air Force's Man In Space Soonest program, had it come to fruition.