Beginning in May 1942, the mission of Fourth Air Force became operational training of units and crews, and the replacement training of individuals for bombardment, fighter, and reconnaissance operations. It received graduates of Army Air Forces Training Command flight schools; navigator training; flexible gunnery schools and various technical schools, organized them into newly activated combat groups and squadrons, and provided operational unit training (OTU) and replacement training (RTU) to prepare groups and replacements for deployment overseas to combat theaters. The Fourth Air Force became predominantly a fighter OTU and RTU organization. Most P-51 Mustang and P-38 Lightning groups were trained by Fourth Air Force primarily due to the proximity of their manufacturing plants in Southern California. By 1944, most of the Operational Training of groups ended, with the command concentrating on RTU training of individual replacements using Army Air Force Base Units (AAFBU) as training organizations at the airfields controlled by Fourth Air Force.