First Lieutenant Greenwell was the pilot of a RB-26C Invader bomber with the 12th Tactical Reconnaissance Squadron, 67th Tactical Reconnaissance Wing. On May 15, 1952, he on a night photo reconnaissance mission over Anju, North Korea, when his aircraft was hit by enemy fire. The crew reported damage and requested direction to the nearest friendly airfield. The plane never made it and the crew was listed as Missing in Action and was presumed dead on December 31, 1953. His name is inscribed on the Courts of the Missing at the Honolulu Memorial.