On September 10, 1965, NASA began the selection process for the fifth astronaut group. From a pool of 351 applicants, NASA selected 159 candidates that met the basic qualifications that required that applicants must be United States citizens born on or after December 1, 1929, and no more than six feet tall. They were also required to have at least 1,000 hours of flight time in a jet aircraft. Mattingly had previously shown little interest and inclination to apply for the astronaut program, however, his views changed when at the Air Force Test Pilot School and his class was offered the chance to apply for either NASA or the USAF Manned Orbiting Laboratory program. Mattingly and Mitchell chose the latter and were rejected. The deadline for applying for the NASA group had passed, however one of their instructors was able to get NASA to accept their applications. On the interview panel the astronaut office representatives were John Young and Michael Collins, at that time in training as prime crew for Gemini 10. Mattingly would later recollect that he was "perplexed" by Young. Collins asked Mattingly how he felt about the F-104, to which Mattingly replied that he thought it was a "fun aircraft" but without worth in combat. Collins appeared to dislike the answer and Mattingly felt he had blown his chance. However, after the conclusion of the selection process, Mattingly was called by Deke Slayton with an offer to become an astronaut.