 At McCook Field, Harris became one of the Army's most important experimental test pilots.On June 8, 1921 Harris became the first pilot to fly a pressurized aircraft when he successfully flew a Dayton-Wright USD-9A with an experimental pressurized cockpit.Two McCook Field test pilots had previously tried to fly the aircraft without success, including test pilot John A. Macready, who held the world altitude record in an oxygen equipped unpressurized aircraft.