Sebille enlisted in the United States Army Air Corps two weeks after the attack on Pearl Harbor by the Empire of Japan. He began flight training in January 1942, in spite of being two months older than the cutoff age of 26, as the desperate need for pilots combined with Sebille's skills as a pilot allowed him to waive the age restriction. During that time he was described as an outstanding pilot and leader, and his maturity was helpful for the younger flight trainees. After completing flight training, Sebille was commissioned as a second lieutenant and assigned to the 450th Bombardment Squadron, 322nd Bombardment Group 3rd Bombardment Wing at MacDill Field, Florida. Sebille flew B-26 Marauder aircraft.