After serving as a private in the infantry during the Spanish-American War (1898), Mitchell received a commission as a second lieutenant in the signal corps. He served in Cuba, the Philippines, and Alaska and in 1909 graduated from the Army Staff College at Fort Leavenworth, Kan. In 1915 he was assigned to the aviation section of the signal corps. During World War I Mitchell became the outstanding U.S. combat air commander, advancing to the rank of brigadier general. In September 1918 he commanded a French-U.S. air armada of almost 1,500 planes—the largest concentration of air power up to that time. In the Meuse-Argonne campaign he used formations of up to 200 planes for mass bombing of enemy targets.

After the war Mitchell was appointed assistant chief of the air service. He became a strong proponent of an independent air force and of unified control of air power, both of which were opposed by the army general staff and the navy. As a result, he became increasingly outspoken in his criticism of the military hierarchy, and, when his term ended in April 1925, he was sent to the remote post of San Antonio, Texas. The climax came in September 1925, when the loss of the navy dirigible Shenandoah in a storm inspired him publicly to accuse the War and Navy departments of “incompetency, criminal negligence, and almost treasonable administration of the national defense.” In December an army court-martial convicted him of insubordination. Sentenced to suspension from rank and duty for five years, he resigned from the army (Feb. 1, 1926).

Nevertheless, Billy Mitchell was awarded many decorations and honours during his lifetime, and in 1946 the U.S. Congress authorized a special medal in his honour; it was presented to his son in 1948 by the chief of staff of the newly created U.S. Air Force.