Duncan was drafted into the Army Air Force (AAF) on July 9, 1942, and was sent to Atlantic City, New Jersey for basic training. Much of the training took place in old hotels commandeered by the military. Duncan soon started contributing cartoons to BEAM, a military publiction, with his feature At Ease. Duncan was next assigned to the USAAF Flight Control Command, in Winston-Salem, North Carolina, where he did training manuel drawings, and another cartoon feature. Duncan was next moved into public relations, working for the Technical Training Command (TTC) at Lowry Field, in Colorado. From there, Duncan was transferred to Tarpon Springs, Florida, where he helped document a survial training experiment for the AAF School of Applied Tactics. Duncan was finally sent to Chanute Field in Illinois, a training facility for those who worked on and flew planes. Duncan's work was done for publicity and training, and showed his breadth as an artist. Soon after his time at Chanute Field, Duncan accepted a position for Stars and Stripes, and was shipped overseas.