When John Glenn was a young man, he didn't plan on a career in space flight. There wasn't any such thing! Glenn was interested in a career in the military, and in aviation in particular. When he entered the Naval Aviation Cadet Program in 1942, he had no way of knowing that this would be the first step on his path to outer space.
Glenn's training in the cadet program prepared him to be a fighter pilot in World War II. Altogether he flew 59 combat missions in that war. After the war, Glenn became a flight training instructor, teaching young men who wanted to become pilots how to fly for the military. Then, in the Korean War, Glenn again flew combat missions as part of a marine fighter squadron. This time he flew a total of 63 missions.
Next, Glenn entered Test Pilot School. He was interested in the field of aircraft design. He also continued to fly. In fact, in 1957 John Glenn set a new speed record as he flew across the continent from Los Angeles to New York.
Glenn's career had been changing all along. As soon as he finished one job, he always looked for a new area to move into. But, in 1959 his career suddenly took off in an exciting new direction. NASA, the National Aeronautics and Space Administration, chose Glenn to be part of a group of men who would train to be astronauts for the Mercury missions.