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.
Walker joined the United States Army in Denver on December 15, 1917. At the University of California's School of Military Aeronautics and at Mather Field near Sacramento, he received flight training. He was granted his Aircrew Badge and commissioned as a temporary second lieutenant in the United States Army Air Service on November 2, 1918.