William Mitchell, byname Billy Mitchell, (born Dec. 29, 1879, Nice, France—died Feb. 19, 1936, New York, N.Y., U.S.), U.S. Army officer who early advocated a separate U.S. air force and greater preparedness in military aviation. He was court-martialed for his outspoken views and did not live to see the fulfillment during World War II of many of his prophecies: strategic bombing, mass airborne operations, and the eclipse of the battleship by the bomb-carrying military airplane.