 After completing his studies at Auburn High School in 1917, Knapp applied for Army officer training, but was turned down due to his age.Instead, he was accepted as an aviation cadet, receiving his wings and being commissioned as a second lieutenant on March 9, 1918.Knapp went to the United Kingdom with his unit, the 92d Aero Squadron (Bombardment), in August 1918, but failed to see action in World War I due to a late shipment of propellers for his unit's Handley Page O/400 bombers.