On August 6, 1945, 74 years ago today, an American bomber dropped the world’s first deployed atomic bomb over the Japanese city of Hiroshima. The explosion wiped out 90% of the city and immediately killed 80,000 people; tens of thousands more would later die of radiation exposure. Three days later, a second B-29 dropped another A-bomb on Nagasaki, killing an estimated 40,000 people. These acts of devastation would have never been possible if it were not for the remarkable Boeing B-29 Superfortress.
In the run-up to World War II, the United States Army Air Corps concluded that the Boeing B-17 Flying Fortress, which would be the United States' primary strategic bomber of the war, would be inadequate for the Pacific Theatre, which required a bomber that could carry a larger payload more than 3,000 miles.
In response, Boeing began work on pressurised long-range bombers in 1938. Boeing's design study for the Model 334 was a pressurised derivative of the Boeing B-17 Flying Fortress with the nose-wheel undercarriage. Although the Air Corps did not have money to pursue the design, Boeing continued development with its own funds as a private venture. In April 1939, Charles Lindbergh convinced general Henry H. Arnold to produce a new bomber in large numbers to counter the Nazi production. In December 1939, the Air Corps issued a formal specification for a so-called "superbomber", that could deliver 20,000 lb of bombs to a target 2,667 miles away and at a speed of 400 mph. Boeing's previous private venture studies formed the starting point for its response to this specification.
Boeing submitted its Model 345 on 11 May 1940, in competition with designs from Consolidated Aircraft’s Model 33, which later became the B-32, Lockheed XB-30 and Douglas XB-31. Douglas and Lockheed soon abandoned work on their projects, but Boeing received an order for two flying prototypes, given the designation XB-29, and an air-frame for static testing on 24 August 1940, with the order being revised to add a third flying aircraft on 14 December. Consolidated continued to work on its Model 33 as it was seen by the Air Corps as a backup in case of problems with Boeing's design.
Boeing received an initial production order for 14 service test aircraft and 250 production bombers in May 1941, this being increased to 500 aircraft in January 1942. The B-29 featured a fuselage design with a circular cross-section for strength, a feature first used in the widely successful Douglas DC3. The need for pressurisation in the cockpit area also led to the B-29 being one of very few American combat aircraft of World War II to have a stepless cockpit design, without a separate windscreen for the pilots.
Manufacturing the B-29 was a complex task. It involved four main-assembly factories: a pair of Boeing-operated plants at Renton, Washington, and Wichita Kansas, a Bell plant at Marietta, Georgia near Atlanta, and a Martin plant at Omaha Nebraska. Thousands of subcontractors were involved in the project.
The first prototype made its maiden flight from Boeing Field, Seattle on 21 September 1942. The combined effects of the aircraft's highly advanced design, challenging requirements, immense pressure for production, and hurried development caused setbacks. The second prototype, which, unlike the unarmed first, was fitted with a Sperry defensive armament system using remote-controlled gun turrets sighted by periscopes, first flew on 30 December 1942, this flight being terminated due to a serious engine fire.
On 18 February 1943, the second prototype, flying out of Boeing Field in Seattle, experienced an engine fire and crashed. The crash killed Boeing test pilot Edmund T. Allen and his 10-man crew, 20 workers at the Frye Meat Packing Plant and a Seattle firefighter. Changes to the production craft came so often and so fast that in early 1944, B-29s flew from the production lines directly to modification depots for extensive rebuilds to incorporate the latest changes. AAF-contracted modification centres and its own air depot system struggled to handle the scope of the requirements. Some facilities lacked hangars capable of housing the giant B-29, requiring outdoor work in freezing cold weather, further delaying necessary modification. By the end of 1943, although almost 100 aircraft had been delivered, only 15 were airworthy. This prompted an intervention by General Hap Arnold to resolve the problem, with production personnel being sent from the factories to the modification centres to speed the availability of sufficient aircraft to equip the first Bomb Groups in what became known as the "Battle of Kansas". This resulted in 150 aircraft being modified in the five weeks between 10 March and 15 April 1944.
The most common cause of maintenance headaches and catastrophic failures was the engines. Although the Wright R-3350 Duplex-Cyclone radial engines later became a trustworthy workhorse in large piston-engine aircraft, early models were beset with dangerous reliability problems. This problem was not fully cured until the aircraft was fitted with the more powerful Pratt & Whitney R-4360 "Wasp Major" in the B-29D/B-50 program, which arrived too late for World War II. Interim measures included cuffs placed on propeller blades to divert a greater flow of cooling air into the intakes which had baffles installed to direct a stream of air onto the exhaust valves. Oil flow to the valves was also increased, asbestos baffles were installed around rubber push rod fittings to prevent oil loss, thorough pre-flight inspections were made to detect unseated valves, and frequent replacement of the uppermost five cylinders, every 25 hours of engine time and the entire engines every 75 hours.
Pilots describe flight after take-off as being an urgent struggle for airspeed, generally, flight after take-off should consist of striving for altitude. Radial engines need airflow to keep them cool, and failure to get up to speed as soon as possible could result in engine failure and risk of fire. One useful technique was to check the magnetos while already on the take-off roll rather than during a conventional static engine run-up before take-off.
In wartime, the B-29 was capable of flight at altitudes up to 31,850 feet, at speeds of up to 350 mph. This was its best defence because Japanese fighters could barely reach that altitude, and few could catch the B-29 even if they did attain that altitude. Only the heaviest of anti-aircraft weapons could reach it, and since the Axis forces did not have proximity fuses, hitting or damaging the aircraft from the ground in combat proved difficult.
The General Electric Central Fire Control system on the B-29 directed four remotely controlled turrets armed with two .50 Browning M2 machine guns each. All weapons were aimed optically with targeting computed by analogue electrical instrumentation. There were five interconnected sighting stations located in the nose and tail positions and three Plexiglas blisters in the central fuselage. Five General Electric analogue computers, one dedicated to each sight, increased the weapons' accuracy by compensating for factors such as airspeed, lead, gravity, temperature and humidity. The computers also allowed a single gunner to operate two or more turrets simultaneously. The gunner in the upper position acted as a fire control officer, managing the distribution of turrets among the other gunners during combat. The tail position initially had two .50 Browning machine guns and a single M2 20 mm cannon. Later aircraft had the 20 mm cannon removed, and sometimes replaced by a third machine gun.
In early 1945 Major General Curtis Lemay, commander of XXI Bomber Command ordered most of the defensive armament and remote-controlled sighting equipment removed from the B-29s under his command. The affected aircraft had the same reduced defensive firepower as the nuclear weapons-delivery intended Silver-plate B-29 airframes and could carry greater fuel and bomb loads as a result of the change. The lighter defensive armament was made possible by a change in mission from high-altitude, daylight bombing with high explosive bombs to low-altitude night raids using incendiary bombs. As a consequence of this requirement, Bell Atlanta produced a series of 311 B-29Bs that had turrets and sighting equipment omitted, except for the tail position, which was fitted with AN/APG-15 fire control radar. This version could also have an improved APQ-7 "Eagle" bombing-through-overcast radar fitted in an aerofoil-shaped radome under the fuselage. Most of these aircraft were assigned to the 315th Bomb Wing, Northwest Field, Guam.
The crew enjoyed, for the first time in a bomber, full-pressurisation comfort. This first-ever cabin pressure system for an Allied production bomber was developed for the B-29 by Garrett AiResearch. The nose and the cockpit were pressurised, but the designers were faced with deciding whether to have bomb bays that were not pressurised, between fore and aft, pressurised sections, or a fully pressurised fuselage with the need to de-pressurise to drop their loads. The solution was a long tunnel over the two bomb bays so as not to interrupt pressurisation during bombing. Crews could crawl back and forth between the fore and aft sections, with both areas and the tunnel pressurised, the bomb bays were not pressurised.
Perhaps the most famous B-29s were the sixty-five examples of the Silver-plate series, which were modified to drop atomic bombs. They were also stripped of all guns, except for those on the tail, in order to have a lighter aircraft. The Silver-plate aircraft were handpicked by Lieutenant Colonel Paul W. Tibbets for the mission, straight off the assembly line at the Omaha plant that was to become Offutt Air Force Base. The Silver-plate bombers differed from other B-29s then in service by having fuel injection and reversible props.
Hiroshima, a manufacturing centre of some 350,000 people located about 500 miles from Tokyo, was selected as the first target. After arriving at the U.S. base on the Pacific island of Tinian, the more than 9,000-pound uranium-235 bomb was loaded aboard a modified B-29 bomber christened "Enola Gay" (after the mother of its pilot, Colonel Paul Tibbets). The plane dropped the bomb, known as “Little Boy”, by parachute at 8:15 in the morning, and it exploded 2,000 feet above Hiroshima in a blast equal to 15,000 tons of TNT, destroying five square miles of the city.
Hiroshima’s devastation failed to elicit immediate Japanese surrender, however, and on August 9 Major Charles Sweeney flew another B-29 bomber, "Bockscar", from Tinian. Thick clouds over the primary target, the city of Kokura, drove Sweeney to a secondary target, Nagasaki, where the plutonium bomb “Fat Man” was dropped at 11:02 that morning. More powerful than the one used at Hiroshima, the bomb weighed nearly 10,000 pounds and was built to produce a 22-kiloton blast. The topography of Nagasaki, which was nestled in narrow valleys between mountains, reduced the bomb’s effect, limiting the destruction to 2.6 square miles.
At noon on August 15, 1945 (Japanese time), Emperor Hirohito announced his country’s surrender in a radio broadcast. The news spread quickly, and “Victory in Japan” or “V-J Day” celebrations broke out across the United States and other Allied nations. The formal surrender agreement was signed on September 2, aboard the U.S. battleship Missouri, anchored in Tokyo Bay.
Following the surrender of Japan B-29s were used for other purposes. A number supplied POWs with food and other necessities by dropping barrels of rations on Japanese POW camps. In September 1945, a long-distance flight was undertaken for public relations purposes: Generals Barney M. Giles, Curtis LeMay, and Emmett O'Donnell Jr. piloted three specially modified B-29s from Chitose Air Base in Hokkaidō to Chicago Municipal Airport, continuing to Washington, D.C., the farthest nonstop distance, 6400 miles, to that date flown by U.S. Army Air Forces aircraft and the first-ever nonstop flight from Japan to Chicago. Two months later, Colonel Clarence S. Irvine commanded another modified B-29, “Pacusan Dreamboat”, in a world-record-breaking long-distance flight from Guam to Washington, D.C., travelling 7,916 miles in 35 hours, with a gross take-off weight of 155,000 pounds. Almost a year later, in October 1946, the same B-29 flew 9,422 miles nonstop from Oahu, Hawaii, to Cairo, Egypt, in less than 40 hours, demonstrating the possibility of routing airlines over the polar ice cap.