The
40th Bombardment Group was constituted in Puerto Rico on 22 November 1940 and activated on 1 April 1941. The unit's operational squadrons (29th, 44th and 45th) were equipped with
Douglas B-18 Bolos then early
Boeing B-17 Flying Fortresses and
Martin B-26 Marauder aircraft to train, and patrol the
Caribbean area, later to provide air defense of the
Panama Canal after the
Japanese attack on Pearl Harbor. With the diminished need for defenses in the Caribbean, the 40th was reassigned back to the United States and redesignated the
40th Bombardment Group (Very Heavy) in November 1943, being assigned to
Pratt Army Airfield,
Kansas and to the first
B-29 Superfortress wing, the 58th Bombardment Wing. At Pratt, the group's squadrons (25th 44th, 45th, and 395th) began transition training on the new aircraft and its new mission. In March 1944, the group left the United States and deployed to a former
B-24 Liberator airfield at
Chakulia, India. In India, the group was assigned to the XX Bombardment Command of the new
Twentieth Air Force. During the week of 15–22 April, no less than five 58th Bomb Wing B-29s crashed near
Karachi all from overheated engines. The entire Wing had to be grounded en route until the cause was found. The cause was traced to the fact that the B-29's R-3350 engine had not been designed to operate at ground temperatures higher than , which were typically exceeded in India. Modifications had also to be made to the aircraft and after these modifications, B-29 flights to India were resumed. From India, the 40th Bomb Group planned to fly missions against Japan from airfields in China. However, all the supplies of fuel, bombs, and spares needed to support the forward bases in China had to be flown in from India over "
The Hump" (the name given by Allied pilots to the eastern end of the
Himalayan Mountains), since Japanese control of the seas around the Chinese coast made seaborne supply of China impossible. Many of the supplies had to be delivered to China by the B-29s themselves. For this role, they were stripped of nearly all combat equipment and used as flying tankers and each carried seven tons of fuel. The Hump route was so dangerous and difficult that each time a B-29 flew from India to China it was counted as a combat mission, The first combat mission by the group took place on 5 June 1944 when the group's squadrons took off from India to attack the Makasan railroad yards at
Bangkok, Thailand. This involved a 2261-mile round trip, the longest bombing mission yet attempted during the war. On 15 June the group participated in the first Army Air Forces attack on the
Japanese Home Islands since the
Doolittle raid in 1942 when it took part in the
bombing of Yawata. Operating from bases in India, and at times staging through fields in China, the group struck such targets as transportation centers, naval installations, iron works, and aircraft plants in
Burma, Thailand, China, Japan,
Indonesia, and
Formosa, receiving a
Distinguished Unit Citation for bombing iron and steel works at
Yawata, Japan, on 20 August 1944. From a staging field in
Ceylon, the 40th mined waters near the port of
Palembang,
Sumatra, in August 1944. The group was reassigned to
Tinian, in the
Marianas February–April 1945, for further operations against Japan with the XXI Bomb Command. The 40th made daylight attacks from high altitude on strategic targets, participated in incendiary raids on urban areas, and dropped mines in Japanese shipping lanes. Received a
Distinguished Unit Citation for attacking naval aircraft factories at
Kure, oil storage facilities at
Oshima, and the industrial area of
Nagoya, in May 1945. Raided light metal industries in
Osaka in July 1945, being awarded another DUC for this mission. After
V-J Day, the group dropped food and supplies to Allied prisoners in Japan, Korea, and Formosa, and took part in show-of-force missions. == Strategic Air Command ==