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