The Stewart family had deep military roots: both grandfathers had fought in the Civil War, and his father had served during both the Spanish-American War and World War I. Jimmy considered his father to be the biggest influence on his life, so it is not surprising that when another war came, another Stewart would be in uniform. With his private pilot's licence in hand and a smattering of flying time, it was also inevitable that Jimmy Stewart would seek to become a military flyer.
Col. Stewart being awarded the Croix de guerre with palm by Lt. Gen. Henri Valin Chief of Staff of the French Air Force, for his role in the liberation of France.

Nearly a year before the December 1941 attack on Pearl Harbour, Stewart attempted to join the United States Army Air Corps, though his enlistment was initially denied due to a weight problem. The USAAF had strict height and weight requirements for new recruits and Stewart was five pounds under the standard. To get up to 148 pounds, he enlisted the help of Metro-Goldwyn-Mayer's muscle man, Don Loomis, who was legendary for his ability to add or subtract pounds in his studio gymnasium. Stewart still came in under the weight requirement and was consequently rejected for being under-weight. Refusing to accept his rejection, he persuaded the AAF enlistment officer to run new tests, this time skipping the weigh-in, with the result that Stewart successfully enlisted in the Army in March 1941. He became the first major movie star to wear a uniform.
Since the United States had yet to declare war on Germany and because of the Army's unwillingness to put celebrities on the front, Stewart was held back from combat duty, though he did earn a commission as a Second Lieutenant and completed pilot training. He later became an instructor pilot for the B-17 Flying Fortress stationed in Albuquerque, NM.
The only public appearances after he went into flight school were limited engagements scheduled by the Air Corps. "Stewart appeared several times on network radio with Edgar Bergen and Charlie McCarthy. Shortly after Pearl Harbour, he performed with Orson Welles, Edward G. Robinson, Walter Huston and Lionel Barrymore in an all-network radio program called 'We Hold These Truths,' dedicated to the 150th anniversary of the Bill of Rights. But mostly, Stewart's days and nights were spent preparing for his upcoming flight tests, ground school and academic examinations for his commission."
"Still, the war was moving on. For the thirty-six-year-old Stewart, combat duty seemed far away and unreachable, and he had no clear plans for the future. But then a rumour that Stewart would be taken off flying status and assigned to making training films or selling bonds called for his immediate and decisive action, because what he dreaded most was the hope-shattering spector of a dead end." So he appealed to his commander, a pre-war aviator, who understood and reassigned him to a unit going overseas.
In August 1943 he was finally assigned to the 445th Bombardment Group in Sioux City, Iowa, first as Operations Officer of the 703rd Bombardment Squadron and then its commander. In December, the 445th Bombardment Group flew its B-24 Liberator bombers to Tibenham, England and immediately began combat operations. While flying missions over Germany, Stewart was promoted to Major. In March 1944, he was transferred as Group Operations Officer to the 453rd Bombardment Group, a new B-24 outfit that had been experiencing difficulties. In 1944, he twice received the Distinguished Flying Cross for actions in combat and was awarded the Croix de Guerre. He also received the Air Medal with three oak leaf clusters. In July 1944, after flying 20 combat missions, Stewart was made Chief of Staff of the 2nd Combat Bombardment Wing of the Eighth Air Force. Before the war ended he was promoted to Colonel, one of only a few Americans to rise from private to colonel in four years.
Jimmy Stewart began flying combat missions on March 31, 1944, and was appointed Operations Officer of the 453rd Bomb Group and, subsequently, Chief of Staff of the 2nd Combat Wing, 2nd Air Division of the 8th Air Force. He ended the war with 20 combat missions and remained in the USAF Reserve where he was later promoted to Brigadier General.
Stewart continued to play an active role in the United States Air Force Reserve after the war, achieving the rank of Brigadier General on July 23, 1959. Stewart did not often talk of his wartime service, perhaps due to his desire to be seen as a regular soldier doing his duty instead of as a celebrity. He did appear on the TV series, The World At War to discuss the October 14, 1943, bombing mission to Schweinfurt — the mission known in USAF history as Black Thursday due to the incredibly high casualties it sustained. Fittingly, he was identified only as "James Stewart, Squadron Commander" in the documentary.
In 1966, Brigadier General James Stewart rode along as an observer on a B-52 Stratofortress bombing run during the Vietnam War, though he did not fly any duty missions during that conflict. Stewart finally retired from the Air Force on May 31, 1968, after 27 years of service. At the time of his B-52 mission, he refused the release of any publicity regarding his participation as he did not want it treated as a stunt for glory, but as his job as an officer in the Air Force Reserve.