After he graduated from the United States Military Academy at West Point in 1907, Arnold served as a second lieutenant of infantry in the Philippine Islands until October 1909.
Soon after his return to the United States, Arnold was detailed to the aviation section of the Signal Corps.
In June 1912, Arnold established an altitude record--6,540 feet (1,993 meters)--and won the Mackay Trophy later in 1912 for a nonstop flight from College Park, Md., to Washington Barracks, D.C., to Fort Myer, Va., and return, a distance of 30 miles (48 km).
In 1916 he was promoted to the rank of captain.
He was the first to fly air mail and to observe artillery fire from a wireless-equipped plane. During World War I, he was assistant director, Office of Military Aeronautics, in command of 30 training schools, 15,000 officers, and 125,000 enlisted men. Arnold was graduated from the Army Industrial College in 1925 and from the Command and General Staff School in 1929. A lieutenant colonel in 1931, he commanded March Field, at Riverside, Calif. Arnold won a second Mackay Trophy in 1935 for his leadership of an Army Alaska flight in 1934. He became assistant chief, Army Air Corps, in 1936; chief, as a major general, in 1938; deputy chief of staff for Air in 1941; and, as a lieutenant general, was appointed commander of all Army Air Forces in March 1942. In March 1943 he was made full general, with all flying services of the Army under his direction until his retirement in 1946.