The 21st Bombardment Squadron (Heavy) flew Consolidated B-24 Liberators in the Aleutian Campaign, where it participated in one of the earliest direct attacks against Japan.

The 21st Bombardment Squadron, Very Heavy flew Boeing B-29 Superfortresses in the strategic bombing campaign against Japan, earning a Distinguished Unit Citation for attacks on the Japanese petroleum industry.

One squadron member, Captain Hilliard A. Wilbanks, was awarded the Medal of Honor for actions that helped rescue Vietnamese Rangers that had been ambushed by Viet Cong forces.

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[4][5] When the Japanese attacked Pearl Harbor, the squadron began antisubmarine patrols over the Gulf of Mexico and dispatched a detachment to Savannah Army Air Base, Georgia[b] to hunt for U-boats off the Atlantic Coast.

On 14 September, along with the 404th Bombardment Squadron of the 28th Group, it conducted the first raid from Adak Army Air Field, an attack on Japanese naval forces at Kiska.

It left Harvard in March 1945 after completing its training and arrived at its combat station, Northwest Field, Guam, on 14 April.

[9][10] After VJ Day, the squadron dropped food and supplies to prisoners of war in Japan, China, Korea and Manchuria.

[12] While initially organizing, it directed air strikes during the Battle of Duc Co[13] The squadron performed visual reconnaissance with light aircraft, flying slowly at low altitude.

By patrolling the same area regularly, squadron forward air controllers grew familiar with the terrain and learned to detect changes that could indicate enemy forces hiding below.

[14] On 27 February 1967, Captain Hilliard A. Wilbanks, one of the squadron's forward air controllers, was flying visual reconnaissance in his Cessna O-1 ahead of a South Vietnamese ranger battalion.

Wilbanks recognized that close air support would not be able to arrive soon enough to help the rangers to withstand the advancing enemy.

As the United States withdrew forces from Vietnam, the squadron moved to Tan Son Nhut Air Base in January 1972 and was inactivated there on 21 February 1973.

It re-equipped with the Fairchild Republic OA-10 Thunderbolt II in 1991, shortly before being inactivated and transferring its mission, equipment and personnel to the 21st Tactical Fighter Squadron in November 1991.

It absorbed the resources of the 3307th Test and Evaluation Squadron, which had been activated on 15 September 1991 and assigned to the 3300th Training Support Group, and which was simultaneously inactivated.