The unit was established in early 1944 at Dalhart Army Air Field, Texas, being formed as a B-29 Superfortress Very Heavy bombardment Group. The unit was formed with three reassigned bomb squadrons (402d, 411th and 430th). The 402d was formed in 1940, becoming a II Bomber CommandB-24 Liberator Replacement Training Unit (RTU); the 411th and 430th having their origins as World War I Air Service Aero Squadrons. The 411th became a II Bomber Command B-17 Flying Fortress RTU; the 430th serving in the Panama Canal Zone as part of the Sixth Air Force before being inactivated. From an original group of 11 officers and 82 enlisted men, the 502d grew to three flying squadrons, a photo lab and thousands of personnel.
In September 1944, the newly formed group was sent to its training station at Grand Island Army Airfield, Nebraska. Due to a shortage of B-29s, the group was equipped with former II Bomber CommandB-17 Flying Fortresses previously used for training heavy bomber replacement personnel. The 502d eventually received Atlanta-built B-29B Superfortresses.
The B-29B was in fact unique, for it was equipped entirely with the only true variant of the B-29 ever manufactured. These aircraft were actually stripped-down versions of the normal B-29, bereft of the General Electric gun system and a variety of other components, in order to save weight and increase bomb-carrying capacity. The resultant unladen weight of 69,000 pounds was a vast improvement, lessening the strain on engines and airframe and enabling the payload to be increased from 12,000 to 18,000 pound ordnance. The only armament on these aircraft was in the tail, where two .50 caliber machine guns were installed. The elimination of the turrets and the associated General Electric computerized gun system increased the top speed of the Superfortress to 364 mph at 25,000 feet and made the B-29B suitable for fast, unescorted hit-and-run bombing raids and photographic missions.
In addition, the B-29Bs of the 502nd were equipped with the new AN/APQ-7 "Eagle" radar sets which gave a much clearer presentation of ground images through a wing-shaped radar vane slung beneath the fuselage. It also gave a biplane effect in appearance. The "Eagle" was the product of the Massachusetts Institute of Technology's Eagle radar development group. It had been designed especially for night missions. During World War II this special antenna and equipment for precision night radar missions was so secret that no B-29s were ever shown with it, and there are no actual official photographs in existence. Missions had to be planned and prepared so that briefing material could be slanted from the radar point of view.
The 502d was deployed to Pacific Theater of Operations (PTO) in late 1944, being assigned to the XXI Bomber Command315th Bombardment Wing in the Northern Mariana Islands; being stationed at Northwest Field, Guam. Upon arrival the group's personnel were engaged in Quonset hut construction. By mid-July most personnel were able to move into the huts from the initial tents which they were assigned on arrival. On 9 July, the group flew a shakedown mission to Truk, carrying about half a normal payload of bombs. The group entered combat on 30 June 1945 when the group bombed enemy installations on Rota.
It then began flying very long range strategic bombardment missions over the Japanese Home Islands, principally attacking targets relating to the enemy's petroleum industry. The 502d was awarded a Distinguished Unit Citation (DUC) for attacks on the coal liquefaction plant at Ube, the tank farm at Amagasaki and the Nippon oil refinery at Tsuchizaki in August 1945. It also conducted night incendiary raids attacking major Japanese cities causing massive destruction of urbanized areas. The group continued strategic bombing raids and incendiary attacks until the Japanese Capitulation in August 1945.
After V-J Day, the 502d dropped supplies to Allied prisoners, participated in show-of-force missions, and flew over Japan to evaluate bombardment damage. In the fall of 1945, the group largely demobilized as part of the "Sunset Project", with some aircraft being sent reclamation on Tinian; others being returned to the United States for storage at aircraft depots in the southwest. By Christmas, the group fleet was reduced to 30 or less planes and the remaining elements of the group was effectively consolidated into the 501st Bombardment Group. Many of the remaining veterans signed for "any conditions of travel" to get home, arriving three weeks later in Oakland, California, where troop trains scattered them for points of discharge close to their homes.
The 502d was inactivated on Guam largely as a paper unit on 15 April 1946.