One of the second group of astronauts dubbed "the New Nine" selected in 1962 for the Gemini program, he piloted the Gemini 4 mission on 3 June 1965 and became the first American to walk in space.
He was killed on 27 January 1967, along with astronauts Gus Grissom and
Roger B. Chaffee, when a fire swept through the capsule of the Apollo I
spacecraft during a rocket test at Cape Kennedy, Florida.