He enlisted in the U.S. Navy on November 5, 1940. He received navy recruit training at Naval Training Station, Norfolk, Virginia and hospital corpsman training at the Norfolk Naval Hospital, Portsmouth, Virginia. In March 1941, Willis was promoted to seaman second class and was briefly assigned to the Naval Hospital, Parris Island, South Carolina, transferring to Naval Air Station, Jacksonville, Florida, in the late spring.
Willis's military career began when he enlisted in the U.S. Navy on November 5, 1940. He underwent training programs at Norfolk, Virginia, including navy recruit training and hospital corpsman training. His promotion to seaman second class in March 1941 led to temporary assignments at various naval facilities before being transferred to Naval Air Station, Jacksonville, Florida.