Aviation Section, U.S. Signal Corps
The Aviation Section, Signal Corps, was the aerial warfare service of the United States from 1914 to 1918, and a direct statutory ancestor of the United States Air Force. It absorbed and replaced the Aeronautical Division, Signal Corps, and conducted the activities of Army aviation until its statutory responsibilities were suspended by President Woodrow Wilson in 1918.
Source: Wikipedia — Aviation Section, U.S. Signal Corps (CC BY-SA 4.0)