Answer
Before becoming an astronaut, Stafford served as a test pilot for the United States Air Force. He was involved in a number of different tests, including the development of the F-100 Super Sabre and the F-105 Thunderchief. He also flew the X-15 rocket plane, which reached an altitude of over 100,000 feet. Stafford's experience as a test pilot helped him to become a successful astronaut.