Assigned to the Air Service, Campbell learned to fly in a Curtiss Jenny aircraft and was later trained in a Nieuport fighter. He was assigned to the famous Pursuit 94th Aero Squadron (the "Hat in the ring" gang) - at this stage flying Nieuport 28 fighters. He was noted for several firsts in his service. He flew the squadron's first patrol along with two other famous aviators, Eddie Rickenbacker and Raoul Lufbery. Due to supply problems, the trio flew their first mission in unarmed planes. His first kill came while flying in an aircraft armed with only one rather than the usual two machine guns.