Bombing Burma’s Bridges: A Daring World War II Mission

Bombing Burma’s Bridges: A Daring World War II Mission

The 7th Bomb Group concentrated on bombing railroad bridges along Japan’s supply line to Burma, including the famous Bridge on the River Kwai.

Here's What You Need to Know: Flying the Hump involved some of the most dangerous missions of the war.

Thirty-five Boeing B-17C Flying Fortress bombers of the 7th Bomb Group happened to be on their way to Asia the morning the Japanese attacked Pearl Harbor. They had been sent to reinforce U.S. Army Air Corps units in the Philippines. Eight of the B-17s arrived at Hickam Field while the Japanese attack was in progress. Three were attacked by Japanese fighters and damaged; one was set afire, becoming the first American-flown B-17 destroyed in World War II. The encounter was one sided; the B-17s’ machine guns had been removed from the bombers to reduce their weight for the long flight over the Pacific. It was a dramatic introduction to World War II.

The 7th BG eventually regrouped in Australia; some of its elements were sent on to join in the futile Allied defense of Java. When Java fell, the group was ordered to India to fly against the Imperial Japanese Army, which had occupied Thailand, Malaya, and Burma. In June 1942, most U.S. heavy bombers based in India were sent to the Middle East to stop German General Erwin Rommel’s advance toward the Egyptian frontier. That reduced the 7th Bomb Group to a single squadron of B-17s and two squadrons of North American B-25 Mitchell medium bombers.

In August 1942, American war planners decided that the 7th would be reconstituted as a heavy bomb group with four squadrons. The new commander of the Tenth Air Force in India, Brig. Gen. Clayton Bissell, did not consider the B-17 suitable for the China-Burma-India Theater (CBI). The Flying Fortress lacked the range required by the long distances in the theater. He asked that the group’s B-17s be replaced by Consolidated B-24 Liberators. Given heavy demand in the European Theater for the Liberator, it would be months before the B-24s reached India. In the meantime, the group flew out of Calcutta and Agra while a new base was prepared for the 7th at Pandaveswar, India, outside Calcutta. 

In 1942, the Allied position in the CBI was precarious. The British Army’s defense of its Burma colony collapsed quickly; surviving British forces retreated to India, which the Japanese were already preparing to conquer. America had just entered the war and was still mustering its forces and establishing the complex logistical network required to support sustained military operations.  

The Japanese Army in Burma had its own logistical challenges. All of its military equipment and supplies had to come from Japan by sea, a voyage of 4,000 miles from the home islands to ports in Burma and Thailand. From those ports to the Burma front lines the journey included an additional 2,000 miles of single-track railroad line. Burma’s heavy jungle offered concealment for the Japanese and made them difficult to target and engage from the air. There were no industrial targets beyond the area of the Burmese capital at Rangoon, and thus an observer wrote, “The air war in Burma became a war against enemy communications and supplies.… The interruption of the movement and transshipment of supplies by sea or land into lower Burma became the primary objective of the 7th Bomb Group.”  

The 7th BGs’ bombers first went after the major targets, the ports at Bangkok and Rangoon, and Japanese shipping heading there via the Gulf of Siam, the Andaman Sea, and the Bay of Bengal. In doing so, the 7th set bombing records. On December 19, 1943, the group flew the longest known mission of the war at that point, to Bangkok, a 14- to 15-hour flight. It also inaugurated new bombing techniques. On November 1, 1944, the campaign to destroy Japanese lines of communication in Burma began, and bridges became the primary targets.  

Author Edward M. Young, in his book B-24 Liberator Units of the CBI,notes, “Bridges were never easy targets. An analysis of the bombing effort during 1943 had shown that the 7th’s Liberators had managed to achieve only one direct hit for every 81 sorties, and these were targets that required direct hits—near misses did little damage to a bridge’s structure…. Part of the solution to the problem of bombing bridges came with the introduction of a new weapon, the AZON bomb.” This was the first American smart bomb, a 1,000-pounder with a radio-controlled tail fin that allowed the bombardier to maneuver the weapon after it had been dropped to correct deflection errors in flight. A flare attached to the rear of the bomb enabled the bombardier to track its fall.

The AZON designation derived from “azimuth only,” which meant the bomb could be steered left or right but lacked pitch control. Much like today’s Joint Direct Attack Munition (JDAM), the AZON control and guidance package was attached to the tail of a standard 1,000-pound bomb. The Eighth Air Force initially tried AZON bombing in Europe in the latter half of 1944, with disappointing results. Apparently Europe’s inclement weather, the rain and the fog, affected AZON guidance. But in the clear, dry weather of Southeast Asia’s hot season, AZON bombs proved ideal for attacking the narrow bridges on the Thai-Burma rail lines. By November 1944, the group had 10 dedicated AZON B-24s fitted with necessary transmitters and antennas and flown by crews specifically trained in the AZON technique. They were assigned to the 7th’s 493rd Bomb Squadron.

The group also developed a new bombing technique. Referred to in some histories as “dive bombing,” it actually was “glide bombing” when performed with a B-24. Young notes, “A form of glide-bombing with a sharp pull-up at the end of the glide could send a bomb directly into a bridge structure instead of it bouncing off as had happened in many low-level attacks…. The pilot would approach the target along its long axis, begin a 20 to 25 degree glide at 1,500 feet, and release the bomb at 500 feet as he pulled out. A toggle switch was fitted to the control column so that the pilot could release the bomb using a special sight designed specifically for that purpose.” On April 24, 1945, a total of 41 Liberators from all four of the 7th’s squadrons were sent out, the 493rd squadron equipped with AZON bombs, the other three prepared for glide bombing. Thirty bridges were destroyed, and 18 were damaged—a spectacular success. 

B-24 bombardier Lieutenant Guilford W. “Chip” Forbes got into the fight at the tail end of the war. He arrived at the 7th’s base at Pandaveswar in January 1945. By that stage of the war, American and British fighter groups in India had established air superiority, but there could still be a lot of flak. Forbes recalled, “Flak could be very heavy, or very light. It depended where you went. On all the missions we flew, we never saw a Japanese fighter.”

All of the missions were long ones and not all targeted railroads. One of the longest of Forbes’ early missions was to the port of Bangkok, his target a drydock that was duly put out of commission. On March 19, 1945, he participated in a mission to the Kra Isthmus, the narrowest part of the Malay Peninsula. All four of the 7th’s squadrons were represented by the 37 B-24s that participated in the raid. The targets were railroad bridges on the Bangkok-Singapore rail line. Each aircraft carried only four bombs; two bomb bays were fitted with extra gas tanks to hold the fuel required by the long flight. Orders were to fly to the target at 400 feet, “a ridiculously low altitude for a B-24,” remembered Forbes. “We heard that the strategy for this raid came right from the top commander in the CBI, Lord Mountbatten, and we griped mightily about his misuse of B-24s.”

Forbes continued, “Lumbering along at 400 feet, a B-24 is an easy target.” Sure enough, Forbes’s B-24 was struck by small-arms ground fire that cut three fuel lines in the bomb bay. “With the bomb bay doors wide open and fuel spilling, our plane was a big-time atomizer,” he added. “We were afraid the next hit would make a spark and the plane would go off like a firecracker.” Forbes and the flight engineer tried to find the leak in the bays with the extra gas tanks as their clothes got saturated, and the pilot started thinking it might be necessary to ditch in the sea. Forbes’ fingers found the leak, finally, and the mission continued. Several significant bridges were among the targets destroyed. The flight was a round trip of 2,700 miles; it took 17½hours, the longest B-24 formation flight made in the CBI. 

On March 22, Forbes’ crew went on a 10-plane raid to Great Coco Island, the north end of the Andaman Island chain, and this time bombed from 3,800 feet. Under normal procedures the Norden bombsight was linked to the autopilot, and course changes on the bomb run were made automatically. But the autopilot was not working; the pilot had to fly manually. Forbes’ primary targets were miniscule from their altitude, two small trailers, about six feet by 10, on which radar equipment was mounted. The targets were about a half mile apart, and each required a separate pass. Forbes had three bombs for three passes. “Everything worked perfectly; two passes, two bullseyes,” he remembered. For the third pass, Forbes picked a building related to the radars “and made another bullseye…. I really believe we could have hit a pickle barrel that day.”