Walker joined the United States Army in 1917. It was after the American entry into World War I. He trained as an aviator and became a flying instructor. In 1920 and after the end of the war, he received a commission in the Regular Army. After service in various capacities he graduated from the Air Corps Tactical School in 1929. Then he served as an instructor there. He supported the creation of a separate air organization for strategic bombardment. He published articles on the subject and becoming part of a clique known as the "Bomber Mafia". They argued for bombers over other forms of military aviation.