As of 2019, who has been the only U.S. military officer to hold the rank of "General of the Air Force"?

Henry H. "Hap" Arnold was born in 1886 and graduated West Point in 1907 as an infantry officer. Unhappy with his duties he sought transfer to the Signal Corps. An encounter in France, where he witnessed an airplane in flight began his interest in flying. In 1911, taking flying lessons at the Wright Brothers' aviation school, he was among the first military aviators in the U.S. military. He served in a variety of posts prior to World War I, assigned to serve in France toward the end of the war, but never had a chance to serve at the front, since he contracted influenza right before the Armistice.

Arnold became Chief of the U.S. Air Corps in 1938, then made commanding general, U.S. Army Air Forces in 1941. The Army Air Forces grew from 800 combat aircraft to nearly 80,000 aircraft by 1944. During his time as commanding general, strategic and intercontinental bombers, jet aircraft and atomic weapons were developed as instruments of modern air power. He was promoted to General of the Army December 21, 1944, in 1949, after his retirement and after the U.S. Army Air Force became a separate service, the U.S. Air Force, he was made General of the Air Force. After the war he was involved in the formation of the RAND Project which evolved into the RAND Corporation.

Arnold suffered a series of heart attacks during the Second World War that ultimately led to his retirement in 1946. He died in 1950.