The squadron was activated in early 1942 as a B-24 Liberator reconnaissance unit. It was redesignated as a heavy bomber Operational Training Unit (OTU), later becoming a Replacement Training Unit (RTU) for deployed combat units, assigned to II Bomber Command, then to III Bomber Command in 1943. The squadron was inactivated in April 1944 when heavy bomber training ended.
The 420th Bombardment Squadron was redesignated as a B-29 Superfortress very heavy bombardment Squadron in August 1944. The squadron was trained under the Second Air Force. Training was considerably delayed due to equipment shortages. They received B-29 aircraft in Kansas in late spring 1945. The ground echelon deployed to Northern Mariana Islands by ship in early August 1945; air echelon remained at the last training base in Kansas after Japanese Capitulation. The ground echelon remained in Marianas supporting other units aircraft and demobilization; the air echelon demobilized with new B-29 aircraft remaining in Kansas, eventually being assigned to postwar units. The entire unit inactivated by December 1945.
On 19 September 1985 the 420th Bombardment Squadron(Very Heavy), (an Inactive Squadron, that was last active 4 January 1946), was consolidated with the 920th Air Refueling Squadron, Heavy. This action was directed by Department of the Air Force Letter DAF/MPM 662q Attachment 1 (Active Units), 19 September 1985.
