On February 20, 1944, Truemper took part in a bombing mission over Leipzig, Germany, as a navigator aboard a B-17G Flying Fortress nicknamed "Ten Horsepower" (AAF Ser. No. 42-31763, markings TU:A). During the mission, the aircraft was attacked by a squadron of German fighter planes. The attack left the bomber severely damaged, the co-pilot dead, the pilot unconscious, and the radio operator wounded. The remaining crewmen were able to fly the plane back to RAF Polebrook, at which point Truemper and the flight engineer, Sergeant Archibald Mathies, volunteered to stay aboard while the others parachuted to safety. After observing the aircraft, their commanding officer determined that it was too badly damaged for the two inexperienced men to land and ordered them to bail out as well. The pilot was still alive and could not be moved, however, and Truemper and Mathies refused to leave him behind. They made two unsuccessful landing attempts before crashing on their third try, killing all three airmen. For this action, both Truemper and Mathies was awarded the Medal of Honor four months later, on June 22, 1944.