Mitchell was accepted into Columbian University (later renamed George Washington University) in Washington, D.C., dropped out to join the United States Army during the Spanish-American War, and eventually graduated from the school While there he was a member of Phi Kappa Psi Fraternity. Upon dropping out of Columbian at age 18, he enlisted in the United States Army as a private and was mustered into Company M of the 1st Wisconsin Infantry Regiment on May 14, 1898. Mitchell was immediately assigned and mobilized into Brigadier General Arthur MacArthur’s command in the Philippines, where MacArthur was placed in charge of the Department of Northern Luzon in the spring of 1899. Mitchell participated in operations against Filipino insurgents in northern and central Luzon at the end of the Spanish-American War and during the Philippine–American War. He quickly gained a commission due to his father’s influence and joined the U.S. Army Signal Corps.

Following the cessation of hostilities, Mitchell remained in the Army. From 1900 to 1904, Mitchell was posted in the District of Alaska as a lieutenant in the Signal Corps. On May 26, 1900, the United States Congress appropriated $450,000 to establish a communications system connecting the many isolated and widely separated U.S. Army outposts and civilian Gold Rush camps in Alaska by telegraph. Along with Captain George C. Brunnell, Lieutenant Mitchell oversaw the construction of what became known as the Washington-Alaska Military Cable and Telegraph System (WAMCATS). He predicted as early as 1906, while an instructor at the Army’s Signal School in Fort Leavenworth, Kansas, that future conflicts would take place in the air, not on the ground.

In 1903, Mitchell married his first wife, Caroline Stoddard. Although the marriage was initially happy, his behavior became more and more erratic, primarily as a result of his heavy drinking. The two had a bitter divorce in 1922, rife with accusations on both sides. Lawyers for Caroline and biographers reported that the marital problems were caused by Billy Mitchell, who became so erratic that his wife even considered sending him to a psychiatrist. Caroline won custody of the children and got a decent alimony.

A member of one of Milwaukee’s most prominent families, Billy Mitchell was probably the first person with ties to Wisconsin to see the Wright Brothers plane fly. In 1908, as a young Signal Corps officer, Mitchell observed Orville Wright’s flying demonstration at Fort Myer, Virginia. Mitchell took flight lessons at the Curtiss Aviation School at Newport News, Virginia.

In March 1912, after assignments in the Philippines that saw him tour battlefields of the Russo-Japanese War and conclude that war with Japan was inevitable one day, Mitchell was one of 21 officers selected to serve on the General Staff—at the time, its youngest member at age 32. He appeared in August 1913 at legislative hearings considering a bill to make Army aviation a branch separate from the Signal Corps and testified against the bill. As the only Signal Corps officer on the General Staff, he was chosen as temporary head of the Aviation Section, U.S. Signal Corps, a predecessor of the present day United States Air Force, in May 1916, when its head was reprimanded and relieved of duty for malfeasance in the section. Mitchell administered the section until the new head, Lieutenant Colonel George O. Squier, arrived from attaché duties in London, England, where World War I was in progress, then became his permanent assistant. In June, he took private flying lessons at the Curtiss Flying School because he was proscribed by law from aviator training by age and rank, at an expense to himself of $1,470 (approximately $33,000 in 2015). In July 1916, he was promoted to major and appointed Chief of the Air Service of the First Army.

When the United States declared war on Germany on April 6, 1917, Mitchell was in Spain en route to France as an observer. He arrived in Paris on April 10, and set up an office for the Aviation Section from which he collaborated extensively with British and French air leaders such as General Hugh Trenchard, studying their strategies as well as their aircraft. On April 24, he made the first flight by an American officer over German lines, flying with a French pilot. Before long, Mitchell had gained enough experience to begin preparations for American air operations. Mitchell rapidly earned a reputation as a daring, flamboyant, and tireless leader. In May, he was promoted to lieutenant colonel. He was promoted to the temporary rank of colonel on October 10, 1917, to rank from August 5.

In September 1918, he planned and led nearly 1,500 British, French, and Italian aircraft in the air phase of the Battle of Saint-Mihiel, one of the first coordinated air-ground offensives in history. He was elevated to the rank of (temporary) brigadier general on October 14, 1918, and commanded all American air combat units in France. He ended the war as Chief of Air Service, Group of Armies, and became Chief of Air Service, Third Army after the armistice.

Mitchell returned to the United States in January 1919; it had been widely expected throughout the Air Service that he would receive the post-war assignment of Director of Air Service. Instead, he returned to find that Maj. Gen. Charles T. Menoher, an artilleryman who had commanded the Rainbow Division in France, had been appointed director on the recommendation of his classmate General Pershing, to maintain operational control of aviation by the ground forces.

Mitchell received appointment on February 28, 1919, as Director of Military Aeronautics, to head the flying component of the Air Service, but that office was in name only as it was a wartime agency that would expire six months after the signing of a peace treaty. Menoher instituted a reorganization of the Air Service based on the divisional system of the AEF, eliminating the DMA as an organization, and Mitchell was assigned as Third Assistant Executive, in charge of the Training and Operations Group, Office of Director of Air Service (ODAS), in April 1919. He maintained his temporary wartime rank of brigadier general until June 18, 1920, when he was reduced to lieutenant colonel, Signal Corps (Menoher was reduced to brigadier general in the same orders).

He returned from Europe with a fervent belief that within a near future, possibly within ten years, air power would become the predominant force of war, and that it should be united entirely in an independent air force equal to the Army and Navy. He found encouragement in a number of bills before Congress proposing a Department of Aeronautics that included an air force separate from either the Army or Navy, primarily legislation introduced concurrently in August 1919 by Senator Harry New of Indiana and Representative Charles F. Curry of California, influenced by the recommendations of a fact-finding commission sent to Europe under the direction of Assistant Secretary of War Benedict Crowell in early 1919 that contradicted the findings of Army boards and advocated an independent air force.

Mitchell believed that the use of floating bases was necessary to defend the nation against naval threats, but the Chief of Naval Operations, Admiral William S. Benson, had dissolved Naval Aeronautics as an organization early in 1919, a decision later reversed by Assistant Secretary of the Navy, Franklin D. Roosevelt. However, senior Naval Aviators feared that land-based aviators in a “unified” independent air force would no more understand the requirements of sea-based aviation than ground forces commanders understood the capabilities and potential of air power, and vigorously resisted any alliance with Mitchell.

When the Army was reorganized by Congress on June 4, 1920, the Air Service was recognized as a combatant arm of the line, third in size behind the Infantry and Artillery. On July 1, 1920, Mitchell was promoted to the Regular Army (i.e., permanent) rank of colonel in the Signal Corps, but also received a recess appointment (as did Menoher) on July 16 to become Assistant Chief of Air Service with the rank of brigadier general. On July 30, 1920, he was transferred and promoted to the permanent rank of colonel, Air Service, with date of rank from July 1, placing him first in seniority among all Air Service branch officers. On March 4, 1921, Mitchell was appointed Assistant Chief of Air Service by new President Warren G. Harding with consent of the Senate. On April 27, Mitchell was reappointed as a brigadier general with date of rank retroactive to July 2, 1920.

The Navy reluctantly agreed to the demonstration after news leaked of its own tests. To counter Mitchell, the Navy had sunk the old battleship Indiana near Tangier Island, Virginia, on November 1, 1920, using its own airplanes. Daniels had hoped to squelch Mitchell by releasing a report on the results written by Captain William D. Leahy stating that, “The entire experiment pointed to the improbability of a modern battleship being either destroyed or completely put out of action by aerial bombs.” When the New-York Tribune revealed that the Navy’s “tests” were done with dummy sand bombs and that the ship was actually sunk using high explosives placed on the ship, Congress introduced two resolutions urging new tests and backed the Navy into a corner.

In February 1921, at the urging of Mitchell, who was anxious to test his theories of destruction of ships by aerial bombing, Secretary of War Newton Baker and Secretary of the Navy Josephus Daniels agreed to a series of joint Army-Navy exercises, known as Project B, to be held that summer in which surplus or captured ships could be used as targets.

On May 1, 1921, Mitchell assembled the 1st Provisional Air Brigade, an air and ground crew of 125 aircraft and 1,000 men at Langley Field in Hampton, Virginia, using six squadrons from the Air Service:

On July 20, 1921, the Navy brought out the ex-German World War I battleship, Ostfriesland. On the scheduled day, 230, 550, and 600 lb (270 kg) bomb attacks by Navy, Marine Corps, and Army aircraft settled the Ostfriesland three feet by the stern with a five-degree list to port. She was taking on water. Further bombing was delayed a day, the Navy claiming due to rough seas that prevented their Board of Observers from going aboard, the Air Service countering that as the Army bombers approached, they were ordered not to attack. Mitchell’s bombers were forced to circle for 47 minutes, as a result of which they dropped only half their bombs, and none of their large bombs.

The fact of battleship sinking was indisputable, and Mitchell repeated the performance twice in tests conducted with like results on the U.S. pre-dreadnought battleship Alabama in September 1921, and the battleships Virginia and New Jersey in September 1923. The latter two ships were subjected to teargas attacks and hit with specially designed 4,300 lb (2,000 kg) demolition bombs.

In 1922, while in Europe for General Patrick, Mitchell met the Italian air power theorist Giulio Douhet and soon afterwards an excerpted translation of Douhet’s The Command of the Air began to circulate in the Air Service. In 1924, Gen. Patrick again dispatched him on an inspection tour, this time to Hawaii and Asia, to get him off the front pages. Mitchell came back with a 324-page report that predicted future war with Japan, including the attack on Pearl Harbor. Of note, Mitchell discounted the value of aircraft carriers in an attack on the Hawaiian Islands, believing they were of little practical use because they were incapable of operating effectively on the high seas, nor capable of delivering “sufficient aircraft in the air at one time to insure a concentrated operation.” Mitchell believed instead, a surprise attack on the Hawaiian Islands would be conducted by land-based aircraft operating from islands in the Pacific. His report, published in 1925 as the book Winged Defense, foretold wider benefits of an investment in air power, believing it at the time, and for the future, “a dominating factor in the world’s development”, both for national defense and economic benefit. Winged Defense sold only 4,500 copies between August 1925 and January 1926, the months surrounding the publicity of the court martial, and so Mitchell did not reach a wide audience.

In March 1925, when Mitchell’s term as Assistant Chief of the Air Service expired, he reverted to his permanent rank of colonel and was transferred to San Antonio, Texas, as air officer to a ground forces corps. Although such demotions were not unusual in demobilizations (Patrick himself had gone from major general to colonel upon returning to the Army Corps of Engineers in 1919), the move was widely seen as punishment and exile, since Mitchell had petitioned to remain as Assistant Chief when his term expired, and his transfer to an assignment with no political influence at a relatively unimportant Army base had been directed by Secretary of War John Weeks.

In response to the Navy’s first helium-filled rigid airship Shenandoah crashing in a storm in September 1925, killing 14 of the crew, and the loss of three seaplanes on a flight from the West Coast to Hawaii, Mitchell issued a statement accusing senior leaders in the Army and Navy of incompetence and “almost treasonable administration of the national defense.” In October 1925, a charge with eight specifications was proffered against Mitchell on the direct order of President Calvin Coolidge, accusing him of violation of the 96th Article of War, an omnibus article that Mitchell’s chief counsel, Congressman Frank Reid, declared to be “unconstitutional” as a violation of free speech. The court martial began in early November and lasted for seven weeks.

However, the court found the truth or falsity of Mitchell’s accusations to be immaterial to the charge and on December 17, 1925, found him “guilty of all specifications and of the charge”. The court suspended him from active duty for five years without pay, which President Coolidge later amended to half-pay. The generals’ ruling in the case wrote, “The Court is thus lenient because of the military record of the Accused during the World War.” MacArthur (who himself in 1951 was removed from duty for similar reasons) later said he had voted to acquit, and Fiorello La Guardia said that MacArthur’s “not guilty” ballot had been found in the judges’ anteroom. MacArthur felt “that a senior officer should not be silenced for being at variance with his superiors in rank and with accepted doctrine.”

Mitchell resigned instead on February 1, 1926, and spent the next decade writing and preaching air power to all who would listen. However, his departure from the service sharply reduced his ability to influence military policy and public opinion.

In 1926, Mitchell made his home with his wife Elizabeth at the 120-acre (0.5 km) Boxwood Farm in Middleburg, Virginia, which remained his primary residence until his death. He died of a variety of ailments, including a bad heart and an extreme case of influenza, in a hospital in New York City on February 19, 1936, at the age of 56, and was buried at Forest Home Cemetery in Milwaukee, Wisconsin. None of Mitchell’s children from his first marriage attended the funeral. His widow, Elizabeth, married Thomas Bolling Byrd, the brother of Virginia governor Harry F. Byrd Sr. and explorer Richard E. Byrd.

Mitchell viewed the election of his one-time antagonist Franklin D. Roosevelt as advantageous for air power, and met with him early in 1932 to brief him on his concepts for a unification of the military in a Department of Defense. His ideas intrigued and interested Roosevelt. Mitchell believed he might receive an appointment as Assistant Secretary of War for Air or perhaps even Secretary of War in a Roosevelt administration, but neither prospect materialized.

Mitchell’s concept of a battleship’s vulnerability to air attack under “war-time conditions” was vindicated after his death. Air power was first shown to be decisive against a capital ship in war conditions during the Spanish Civil War: on 29 May 1937, Republican Government bombers attacked and damaged the German pocket battleship Deutschland. This new dimension for aerial warfare preceded the attack on Taranto and Pearl Harbor by a good margin.

Mitchell’s son, John, joined the Army in 1941. Promoted to first lieutenant in the 4th Armored Division, he died from a blood infection in 1942. Mitchell’s first cousin, the Canadian George Croil, went on to secure an autonomous status for the Royal Canadian Air Force and in 1938 became its first Chief of the Air Staff.

There has been some confusion regarding Mitchell’s medal being a Medal of Honor instead of a Congressional Gold Medal, because it was erroneously listed as a Medal of Honor in the Committee on Veterans’ Affairs report of 1979 which is often used as a modern index of Medal of Honor listings. According to the Army’s Center of Military History, “it seems apparent that the intention was to award the Gold Medal rather than the Medal of Honor,” but the Center included Mitchell’s award because of the error on the Senate report. The Senate error was a consequence of a drafting mistake when the bill was in committee. The House Committee on Military Affairs confused the Medal of Honor with the Congressional Gold Medal in its first draft of the bill, and then retroactively amended the text to remove “a Medal of Honor” and replace it with “a gold medal,” but neglected to correct the title of the bill. However, the Committee clarified that “the legislation under consideration does not authorize an award of the Congressional Medal of Honor,” which clearly settled the matter. In spite of these verifiable facts, the U.S. Air Force still lists Mitchell as a Medal of Honor recipient, even though he has been removed from the official list published online by the Department of Defense. According to one author, the Air Force’s continued representation that Mitchell is a Medal of Honor recipient is “inexplicable, since Congress lists the award as a Gold Medal, the Air Force formally participated in the Gold Medal’s design, and the National Museum of the Air Force currently possesses the replica Gold Medal in question.” The National Museum of the Air Force displays Mitchell’s Gold Medal publicly, with the caption that “This is the Special Congressional Medal of Honor awarded posthumously to Gen. Billy Mitchell in 1946. This medallion, the only one of its kind in existence, was sculpted by Erwin F. Springweiler, and was struck by the Philadelphia Mint.” Since the medal in question is on public display, it is easily verifiable as not being a Medal of Honor. Several Medal of Honor historians have also published on this subject due to repeated confusion over Mitchell’s award.