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The First Nuclear Bomber — Inside the Evolution of Boeing’s B-29 Superfortress

ON AUGUST 6, 1945, the United States dropped an atomic bomb on Hiroshima, the first such weapon to be used in wartime. The plane that undertook the historic mission was a four-engine, heavy bomber dubbed “Enola Gay,” one of nearly 4,000 Boeing B-29 Superfortresses built during the Second World War.

For more than a year, formations of B-29s, armed with both conventional and incendiary bombs, had been striking the Japanese Home Islands with devastating results. The mission by Enola Gay, and the raid on Nagasaki by the Superfortress dubbed “Bockscar” three days later, signalled the dawn of the nuclear age.

The B-29 itself was the result of over 10 years of planning for a possible war in the Pacific. Indeed, the origins of this super-bomber dated back to 1933, when War Plan Orange, the long-standing American strategy for fighting the Empire of Japan, was radically updated.

The plan, which was revised to incorporate long-range strategic bombing of the Japanese home islands, highlighted the need for a heavy bomber with the range to cross thousands of miles of ocean. In 1933, the United States Army Air Corps’ principal bomber, the Martin B-10, had a combat radius of less than 650 miles with a 2,000 pound bomb load. Clearly, a new bomber capable of flying farther would be necessary.

In the mid-1930s, the United States’ main frontline bomber was the B-18 Bolo. It soon became clear, larger planes capable of striking targets thousands of miles away would be needed in a future war. (Image source: WikiMedia Commons)

‘Experimental Bomber, Long Range’

In early 1934, the USAAC announced two new competitions. One requirement called for a bomber capable of carrying 2,000 pounds of bombs with a combat radius of 1,000 miles and a speed of 200 mph. This led to the development and production of both the B-17 Flying Fortress and obsolescent B-18 Bolo bombers.

The second requirement called for a bomber capable of carrying 2,000 pounds of bombs with a combat radius of 2,500 miles at 200 mph.

The Boeing Airplane Company and the Glen L. Martin Company both responded to the second requirement for a superbomber.

Martin offered its Model 145 bomber, a large bomber of conventional layout. Martin intended to use four new liquid-cooled Allison V-1710 engines which provided a much more streamline profile. The designers also incorporated Fowler flaps to reduce the take-off and landing speed needed for the massive load of up to 65,000 pounds gross weight, almost four and half times the weight of current bombers.

The XB-15 was the first bomber with a ranger of over 3,000 miles. (U.S. Air Force photo)

When the initial design was rejected as underpowered, Martin radically redesigned the Model 145 adding two pusher engines to the four tractors. The body was also changed from a standard single-fuselage arrangement into a twin-boom tail.

The wingspan was increased to 173 feet, almost two and half times the size of the B-10. Along with the extra power and size came extra weight, pushing the maximum weight up to 105,000 pounds. The USAAC decided that the Martin Model 145, which was designated XB-16, was too big and expensive. The project was cancelled in 1935, before any examples were built.

Boeing was more successful in the bid for the superbomber contract with its Model 294. A large, mid-wing plane with four engines, Boeing originally planned to use the same Allison V-1710 engines proposed by Martin. The Model 294’s wing was so thick it was even possible for a crew member to service the engines from inside while in flight. The proposed defensive armament of six machine guns was the most ever proposed on a single airplane up to that point.

With a range of 5,000 miles, it would fly beyond the range of fighter escort and needed to defend itself. The USAAC was impressed enough to issue a contract for a full design study on June 28, 1934. The new bomber was designated XBLR-1 for Experimental Bomber, Long Range. By mid-1935, it was clear that the Boeing design was superior to Martin’s, and the USAAC issued a contract on June 29, 1935 for a single prototype, serial number 35-277.

Boeing constructed the XBLR-1 simultaneously with the B-17, but construction took much longer due to the complexity of the aircraft. It was redesignated XB-15 long before it was completed. When the Allison engines were not ready in time, Boeing reverted to 1,000 horsepower (hp) Pratt & Whitney R-1830 radial engines.

The XB-15 finally took to the air on October 15, 1937. It was at that time the largest and heaviest aircraft ever built in the United States with a wingspan of 149 feet and weighing in at over 65,000 pounds gross. Unfortunately, the R-1830 engines did not provide enough power for a bomber of such massive dimensions. While it certainly had the lifting power and range — the XB-15 carried a 71,000-pound payload to 8,200 feet, and it carried a 4,400-pound payload over a distance of 3,100 miles — but it could only achieve a cruising speed of 171 mph.

With the completion of testing in August 1938, the XB-15 was turned over to the 2nd Bomb Group, and no further aircraft were ordered.

The XB-19 tested the limits of long range aviation technology. (Image source: WikiMedia Commons)

Going the distance: Project D

Still unready to place orders for the XBLR-1, on February 5, 1935, the USAAC initiated a top secret, proof-of-concept project for another even longer-ranged bomber, codenamed Project D.

The Douglas Aircraft Company and the Sikorsky Aircraft Company were approached to submit designs, designated XBLR-2 and XBLR-3 respectively. After inspecting the competing designs and mockups, the USAAC decided that the Douglas design was the winner in March 1936. Unfortunately, limited research and development funding delayed construction of a prototype until early 1938, by which time, it was designated XB-19.

The prototype, serial number 38-471, would not fly until June 1941. With a wingspan of 212 feet and a gross weight of 140,000 pounds, it dwarfed even the XB-15. This aircraft had a maximum speed of over 200 mph (265 mph when it was supplied with more powerful engines) and a combat range of 5,200 miles. Although never put into production, it provided a wealth of data, which was applied to the design of future long range super bombers.

When it became clear that the USAAC would not order the B-15, Boeing sought to remedy its shortcomings with an even newer design, the Model 316. This model was a slightly refined version of the XB-15 powered by 1,400 hp Pratt & Whitney R-2180 Twin Hornet radial engines. Boeing offered several versions of the Model 316, and the high-winged Model 316D attracted a USAAC order in June 1938, for two airframes designated Y1B-20. Unfortunately for Boeing, cooperation between the army and navy on war planning broke down around the same time and the air corps’ requirements for super long-range bombers were put on hold. The Y1B-20s were cancelled before construction began.

Meanwhile, the USAAC had asked Boeing to explore a pressurized version of the B-17. In March 1938, Boeing created the Model 322 design study featuring a new, large diameter fuselage with standard B-17 wings and tail. Despite its potential, the army lacked research funds to pursue the project further. Boeing however believed in the potential of the aircraft and decided to continue work at its own expense.

During the progress of the design, the Boeing bomber grew to over 65,000 pounds. With the Model 334A of July 1939, the Boeing design team adopted a high aspect wing, which provided the ability to carry 2,000 pounds of bombs over a 5,000-mile range at speeds up to 350 mph. Boeing felt it had a winner with this design, and it funded a mockup in December 1939, despite the lack of a requirement.

Conflict spurs development of the super bomber

Events in Europe and China soon drew the army’s attention away from its conflict with the navy over war planning.

General Henry H. “Hap” Arnold established a special committee to look at the long-term needs of the USAAC. The group submitted a report in June 1939 recommending new long-range medium and heavy bombers. With the outbreak of World War II in Europe, Arnold was able to get approval in December 1939, to contract for very long-range heavy bombers from four leading aircraft companies. By January 1940, the USAAC issued formal requirements for the new super bombers to the Consolidated Aircraft Company, Lockheed Aircraft Company, Douglas and Boeing. The requirement called for a plane with a speed of 400 mph and a range of 5,333 miles with a bomb load of 2,000 pounds.

Boeing had an immediate head start on the competition thanks to its self-funded studies in 1938-39. Having begun work on the Model 341 in August of 1939, the company believed it could offer an aircraft featuring a new high-lift wing spanning over 124 feet. Four 2,000 hp Pratt & Whitney R-2800 radial engines were expected to provide a maximum speed of over 400 mph. The predicted range would be 7,000 miles with one ton of bombs.

In order to meet the revised requirements, Boeing reworked their design into the Model 345 using Wright R-3350 engines. With ten machine guns and a 20-mm tail cannon, the new design met all the defensive requirements, as well as the range, payload and speed requirements. Boeing had its design into the army by May 1940 and had obtained a contract for additional studies and wind tunnel testing the following month.  The new Boeing design was designated the XB-29.

The Lockheed XB-30 was similar in size to the XB-29, but Lockheed realized it was too far behind to compete with Boeing and withdrew from the competition. Lockheed eventually developed the XB-30 design into the famous Constellation airliner.

The Douglas XB-31 progressed a little further than the Lockheed design. Based on experience with the XB-19, Douglas proposed a larger and heavier aircraft with a 207-foot wingspan carrying a maximum weight of 198,000 pounds.

This massive aircraft would require four 3,000 hp Pratt & Whitney R-4360 engines with 28 cylinders each. The complexity of the design meant that the XB-31 was still in the design stages by the time the B-29 was ordered into production in September 1941. Shortly thereafter, the XB-31 was cancelled.

The Consolidated XB-32 fared much better than the XB-30 or XB-31. Initially, the XB-32 was an enlarged version of the B-24 Liberator with a 135-foot wingspan and a gross weight of over 100,000 pounds. This aircraft would use 2,200 hp Wright R-3350-13 Cyclone engines to achieve a top speed of 376 mph. The similarity of the design to the successful B-24 inspired confidence and helped speed the development.

The XB-32 was actually airborne before the XB-29. The army wanted to pursue both bombers in parallel in case one design failed. Unfortunately, an early crash of the first prototype XB-32 caused a significant delay in the program, leaving Boeing in the lead with the dominant design. Eventually however, 115 production B-32 Dominators were ordered after Consolidated had substituted a tall single rudder for the twin rudders used on the prototypes. These aircraft were used primarily as trainers and in other non-combat roles.

The army had been impressed with the Boeing design from the start. The company was the first to receive an order for two prototypes in August 1940, a month ahead of the XB-32 contract. The design looked so promising that the army decided to procure the B-29 even before the prototype was completed.

An order for 250 B-29s was confirmed in September 1941, a full year before the aircraft even left the ground. With America’s entry into World War II, the original order was increased to over 1,500 aircraft by March 1942. The XB-29 (41-0002) first flew, two weeks after the XB-32, on September 21, 1942. Aside from overheating problems with the R-3350 engines, the performance and handling of the B-29 were excellent.

Early models of the B-29 in olive drab livery. (Image source: WikiMedia Commons)

Birth of the nuclear bomber

By 1944, the B-29 was the only bomber with sufficient range to attack Japan from existing bases. The first B-29 bombing raid occurred in June of that year. This mission was over 2,200 miles round trip making it the longest bombing mission attempted up to that time.

Later in August, B-29s staged out of Ceylon to bomb oil facilities on Sumatra. This 19-hour mission covered 4,033 miles, and it was the longest American bombing mission of the war. The B-29 had earned its moniker of Superfortress.

By the end of 1944, B-29 bases shifted to the Marianas Islands, and five separate B-29 bombardment wings rained bombs on Japan until two B-29s abruptly ended the war and ushered in the nuclear era. The end result of the original super bomber competition carried America through the start of the Cold War and through the entire Korean War.