 Walker enlisted in the United States Army in Denver , on 15 December 1917 . He received flight training at the University of California 's School of Military Aeronautics and at the pilot training base at Mather Field , near Sacramento , California . He was awarded his Aircrew Badge and commissioned as a temporary second lieutenant in the United States Army Air Service on 2 November 1918 . He then attended the Flying Instructor 's School at Brooks Field in San Antonio , Texas , and became an instructor at the flight training center at Barron Field . In March 1919 , he was posted to Fort Sill as an instructor at the Air Service Flying School . During 1918 , the School for Aerial Observers and the Air Service Flying School were built at nearby Post Field , where Walker spent the next four years as a pilot , instructor , supply officer , and post adjutant . 
