 The 21st Bombardment Group became the B-25 Operational training unit at Columbia on 21 April 1942, until the unit was reassigned organizationally to Key Field, Mississippi on 22 May.The 21st was replaced by the 309th Bombardment Group, which exchanged designations with the 21st, being reassigned from Key Field.The 329th Bomb Group (and its successor designations) was the major operational training unit (OTU) at Columbia AAB during World War II, providing crew and replacement training in B-25s until 1 May 1944 when the 309th was re-designated as the 329th Bombardment Group.