From the XBLR-1 to the B-29
Development of America's Global "Superbomber"
by Randy Wilson
Copyright © 1995 by the Confederate Air Force and Randy Wilson. All rights reserved.
With the recent celebration of the fiftieth anniversary of the end of World War II, much has been written in aviation publications about the Boeing B-29 Superfortress the plane that dropped the atomic bomb and ended the war. Less told, however, is the story of why and how only America built a successful very long range strategic bomber in the Second World War.
After it's experiences in World War I, the United States turned inward, some termed it isolationism, and bombing aircraft could be justified and funded solely for defense. This attitude persisted until the early 1930s, when the expansion of Imperial Japan into China and Manchuria and the rise of Hitler and rearmament of Germany caused a few military planners to envision a global war with enemies across both the Atlantic and Pacific oceans.
In April of 1934, the Army Air Corps issued a requirement for a bomber with a range of 5,000 miles, a bombload of 2,000 lbs and speed of 200 mph. Boeing proposed a design, initially designated the XBLR-1 (Experimental Bomber, Long Range), powered by four 1,600 hp Allison XV-3420 24-cylinder liquid-cooled engines. This engine was not ready until 1940. The single XBLR-1, by then redesignated XB-15, first flew in October 1937, powered by four Pratt & Whitney R-1830 radials of 1,000 hp each. Even though underpowered, the XB-15 still set payload and distance records in 1939.
Allison's V-3420 engine was basically two 12-cylinder V-1710 engines, which powered the P-38, P-39 and P-40, joined in a common crankcase. When it finally ran in 1940, the V-3420 produced 3,000 hp. It was also chosen to power Douglas' four-engine heavy bomber, the XBLR-2, later redesignated XB-19. The single prototype, however, was powered by four Wright R-3350-5 18-cylinder, twin-row radial engines, producing 2,000 hp each. Even with these more powerful engines, the XB-19 lacked a low-drag, high-speed wing and other aerodynamic refinements. Although it was not a success, the XB-19 served out the remainder of the war as a flying laboratory and cargo transport.
Despite the failure of the XB-15, a scaled-back version of its design resulted in the famous Boeing B-17 Fortress, which first flew in 1935. Both the B-17 and its stablemate, the Consolidated B-24, were true strategic bombers, but by late 1939, with the outbreak of the war in Europe, a few American air planners saw the need for an even longer-range, global bomber.
In November 1939, the Army Air Corps' General "Hap" Arnold issued a specification for a very-long-range bomber superior to the B-17 and B-24. In April 1940, Boeing, Lockheed, Douglas and Consolidated each submitted design studies, and their proposals were designated XB-29, XB-30, XB-31 and XB-32 respectively. After a short while, both Lockheed and Douglas withdrew their designs. Consolidated's B-32 was eventually produced in small numbers as a backup to the B-29 project.
Since early 1938, Boeing had been planning an improved, pressurized, tricycle-geared version of the B-17. In January 1940, the Army issued further requirements for what was now being called the "Superbomber" a plane with a range of over 5,000 miles with a 2,000 pound bombload and speed of 400 mph. Boeing submitted its final design for the XB-29 to the Army on May 11, 1940, and a contract dated Sept. 6, 1940 authorized two prototypes to be built, later increased to three.
The XB-29 was powered by four Wright R-3350-13 of 2,200 hp, each fitted with two turbo-superchargers for greater power at high altitudes. The first flight was made September 21, 1942, but the second prototype was lost in February 1943 while trying to land with an engine on fire. This was not the last of the B-29's problems with engine overheating and fires.
The B-29's wing was very long compared to its width or chord, with an aspect ratio of 11.5, producing low-drag at high speeds, but also very high wing loadings. To improve the wing's performance at low speeds, large-area Fowler flaps were fitted, which actually increased the wing's area by twenty percent when lowered. The front and rear cabins in the fuselage were pressurized, allowing the crew to operate at high altitudes without wearing oxygen masks.
But pressurization made manned gun turrets impractical, so all defensive guns, except the tail turret, were controlled remotely from separate gunners positions. Despite being one-third larger and almost twice as heavy, the aerodynamic refinements of the B-29 resulted in it having no more drag than a B-17.
The first service evaluation YB-29s began to roll off the lines at Boeing's Wichita plant in April 1943. Production was also begun by Bell in Marietta, Georgia and by Martin in Omaha, Nebraska. Experience with early production B-29s showed the need for modifications and additional equipment to be combat-ready, but the decision was made not to stop production to make these changes. Thus, from March 10 to April 15, 1944 new B-29s had to be flown to modification centers to be made combat-ready before being sent overseas. This period was know as the "Battle of Kansas".
A total of about 2,181 of the initial B-29 model were built 1,620 by Boeing, 357 by Bell and 204 by Martin, all powered by Wright R-3350-23 engines. The B-29A version which followed had a redesigned wing structure and one foot greater wingspan, redesigned engine nacelles, and four instead of two guns in the forward turret. Boeing built 1,119 B-29As at its Renton, Washington plant. The Ghost Squadron's Superfortress, Fifi, is a Renton-built B-29A, and the only B-29 still flying.
The final production version Superfortresses were 311 Bell-built B-29Bs, which had improved R-3350-51 engines, and all guns removed except the tail turret, to increase both range and speed.
The first B-29 arrived in China in April 1944, and the first raid on Japan was launched from India and advanced bases in China on June 5, 1944. However, difficulties in supplying the Chinese bases over the Himalayan Mountains, the "Hump", limited the number and effectiveness of the raids. Clearly, bases in the Pacific had to be found to carry the airwar to the Japanese home lands in force. Thus, America's final strategy in the Pacific was simple capture islands close enough to Japan to be able to use B-29s to destroy the enemy's ability and will to fight.
The first B-29 landed on Saipan in the Mariana Islands on October 12, 1944. On Nov. 24, the first raid on Japan was launched. By February 1945, airfields on Tinian and Guam were also operational. Flying from these hot Pacific islands increased overheating problems with the B-29's big Wright engines, and crews learned to keep engine runups to a minimum before takeoff. Still, engine fires continued to be a problem. Some reports note that magneto checks were not done until after beginning the takeoff run certainly not the best time to find out you have an ignition problem.
On the night of March 9/10 1945, 334 B-29s took off from the island bases to deliver the first major incendiary attack on Tokyo. The results were devastating, with more than 16 square miles of the city burned. Superfortresses continued to strike Japan from the Marianas with both high explosive and incendiary bombs until the end of the war. The two atomic attacks which finally caused Japan to surrender were carried out by B-29s flying from Tinian.
During World War II, no other country produced a successful global bomber, and one can only wonder where American air power would have been without the XBLR-1 or XB-19, many years before the B-29 was needed to span the vast distances of the Pacific and help end the war.
Note: Performance and weight data for B-17G and B-24J models. Maximum
bombloads for all could be carried for shorter ranges only.
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