During World War I, Doolittle stayed in the United States as a flight instructor and performed his war service at Camp John Dick Aviation Concentration Center ("Camp Dick"), Texas; Wright Field, Ohio; Gerstner Field, Louisiana; Rockwell Field, California; Kelly Field, Texas and Eagle Pass, Texas.
Doolittle's service at Rockwell Field consisted of duty as a flight leader and gunnery instructor. At Kelly Field, he served with the 104th Aero Squadron and with the 90th Aero Squadron of the 1st Surveillance Group. His detachment of the 90th Aero Squadron was based at Eagle Pass, patrolling the Mexican border. Qualifying for retention in the Air Service during demobilization at the end of the war, Doolittle received a Regular Army commission as a 1st Lieutenant, Air Service, on July 1, 1920. Subsequently, he attended the Air Service Mechanical School at Kelly Field and the Aeronautical Engineering Course at McCook Field, Ohio.
Having at last returned to complete his college degree, he earned the Bachelor of Arts from the University of California, Berkeley in 1922, and joined the Lambda Chi Alpha Fraternity. Doolittle was one of the most famous pilots during the inter-war period. In September 1922, he made the first of many pioneering flights, flying a de Havilland DH-4 – which was equipped with early navigational instruments – in the first cross-country flight, from Pablo Beach (renamed Jacksonville Beach), Florida, to Rockwell Field, San Diego, California, in 21 hours and 19 minutes, making only one refueling stop at Kelly Field. The U.S. Army awarded him the Distinguished Flying Cross.
In July 1923, after serving as a test pilot and aeronautical engineer at McCook Field, Doolittle entered the Massachusetts Institute of Technology. In March 1924, he conducted aircraft acceleration tests at McCook Field, which became the basis of his master's thesis and led to his second Distinguished Flying Cross. He received his S.M. in Aeronautics from MIT in June 1924. Because the Army had given him two years to get his degree and he had done it in just one, he immediately started working on his Sc.D. in Aeronautics, which he received in June 1925. He said that he considered his master's work more significant than his doctorate. Following graduation, Doolittle attended special training in high-speed seaplanes at Naval Air Station Anacostia in Washington, D.C.. He also served with the Naval Test Board at Mitchel Field, Long Island, New York, and was a familiar figure in air speed record attempts in the New York area. He won the Schneider Cup race in a Curtiss R3C in 1925 with an average speed of 232 MPH. For that feat, Doolittle was awarded the Mackay Trophy in 1926.
In April 1926, Doolittle was given a leave of absence to go to South America to perform demonstration flights. In Chile, he broke both ankles, but put his P-1 Hawk through aerial maneuvers with his ankles in casts. He returned to the United States, and was confined to Walter Reed Army Hospital for his injuries until April 1927. Doolittle was then assigned to McCook Field for experimental work, with additional duty as an instructor pilot to the 385th Bomb Squadron of the Air Corps Reserve. During this time, in 1927 he was the first to perform an outside loop previously thought to be a fatal maneuver. Carried out in a Curtiss fighter at Wright Field in Ohio, Doolittle executed the dive from 10,000 feet, reached 280 miles per hour, bottomed out upside down, then climbed and completed the loop.