The heaviest production aircraft then built, the B-29 would be the first bomber to have pressurized crew compartments, computerized fire control, the ability to carry a 20,000 pound bomb load and the capability to fly 5,800 miles at an airspeed of 360 miles per hour. Although the B-29 was designed as a high altitude bomber, its greatest successes were at low altitude. Low-flying B-29's laid waste to most militarily important cities in Japan. It also has the distinction of being the first, and to date the only, bomber to have dropped nuclear weapons in combat.

Evolution of the B-29

	Boeing XB-15
	Type: Long range heavy bomber with a crew of ten. Engines: Four Pratt & Whitney R-1830-11 Twin Wasp air cooled radials, rated at 1000 hp for takeoff. Performance: 200 mph at 5000 feet, cruising speed 152 mph at 60 percent power at 6000 feet. Service ceiling 18,900 feet, absolute ceiling 20,900 feet. Range: 3400 miles with 2500 pounds bomb load, maximum range 5100 miles. Weights: 37,300 pounds empty, 65,000 pounds gross, 70,700 pounds maximum. Dimensions: Wingspan 149 feet, length 87 feet, height 18 feet, wing area 2780 square feet. Armament: Two 0.50-inch and four 0.30-inch machine guns. Maximum bomb load was 12,000 pounds

Lessons

Boeing however, learned a great deal from the XB-15 experience. The Model 316 differed from the XB-15 primarily in the relocation of the wing from a low to a high position and by the installation of a tricycle undercarriage. To overcome the low power situation, the Model 316 was to use four 2000 hp Wright R-3350 Cyclone eighteen-cylinder air-cooled radials. These engines were then on the drawing board and expected to be available in late 1939.

In 1938 the Air Corps issued a requirement for a high altitude bomber (30,000ft) capable of delivering a 4000lb payload over a range of 2000 miles. In response Boeing made a series of design changes from the 316 to create the 322 - an adapation of their civil Model 307 Stratoliner with pressurized fuselage and B-17 wings and tail. This evolved into the Model 333A of late 1938 which was to be powered by four 1150 hp Allison V-1710 twelve-cylinder liquid-cooled engines, installed in tandem pairs. Full pressurization of the cabin was considered impractical because of the need to open the bomb bays during high-altitude flight, and it was decided that only the crew areas in the nose and in the mid-fuselage sections were to be pressurized.

The nose and mid-fuselage cabins were to be connected by a pressurized tunnel passing over the bomb bay to allow crew members to change positions during flight. This feature remained on all subsequent Boeing long range bomber design proposals.

Because of the poor high-altitude performance of the Allison engine, variations of the project were proposed using new Wright and Pratt & Whitney radial engines. This led to the Model 333B project of February 1939 to be powered by four Wright engines buried in a thick wing. With a gross weight of 52,000 pounds, the maximum speed was to have been 364 mph at 20,000 feet. The range was to have been 2500 miles with a 2000-pound bomb load.

In March of 1939, the Model 334 was proposed. The wingspan was extended to 120 feet in order to provide enough fuel for a range of 4500 miles. The Pratt & Whitney radials were still buried in the wing. A twin fin-and-rudder was to have been used to allow the installation of tail armament. Gross weight was up to 66,000 pounds and maximum bomb load was 7800 pounds.

Boeing revised the design further in July of 1939 to create the Model 334A. It dropped the buried engines and the twin fin and rudder assembly and replaced them with four nacelle mounted Wright R-3350 radials and a single vertical tail. A high aspect-ratio wing of 135 feet span was to be used. The Boeing proposal had taken on the shape of what was eventually to become the B-29 Superfortress.

While Boeing was developing its design, General "Hap" Arnold, the head of the Army Air Corps, had become alarmed by the growing war clouds in Europe and by the Japanese military campaign in China. He established a special committee chaired by Brigadier General W.G. Kilner to make recommendations for the long term needs of the Army Air Corps. Charles Lindbergh had been a member of the committee and had recently toured German aircraft factories and Luftwaffe bases. He was convinced that Germany was ahead of its potential European adversaries and in some areas the United States. In their June 1939 report, the Kilner committee recommended that several new long-range medium and heavy bombers be developed, proposing that the classic Douhet theory of long range destruction of an enemy's war industries would prevent the USA from becoming involved i another land war in Europe and against Japan.

The outbreak of war in Europe on September 1, 1939, added weight to the committee's recommendations and on November 10, General Arnold requested authorization to contract with major aircraft companies for studies of a Very Long-Range (VLR) bomber capable of carrying any future war well beyond American shores. Approval was granted on December 2, and USAAC engineering officers under Captain Donald L. Putt of the Air Material Command at Wright Field began to prepare the official specification.

In January of 1940, the Army issued the formal requirements for the VLR "superbomber". The requirements called for a speed of 400 mph, a range of 5000 miles and a bomb load of 2000 pounds. This became the basis for Specification XC-218. On January 29, 1940, the War Department formally circulated it to Boeing, Consolidated, Douglas and Lockheed. The official specification was revised in April to incorporate the lessons learned in early European wartime experience and now included more defensive armament, armor and self-sealing tanks.

The Boeing company had been thinking along similar lines. In August 1939, Boeing started work on the Model 341 project, which featured a new high-lift airfoil for a high aspect-ratio wing of 124 feet span. The Model 341 offered a maximum speed of 405 mph at 25,000 feet. It was to have been powered by four 2000 hp Pratt & Whitney R-2800 radials, weighed 85,000 pounds with a range of 7000 miles carrying 2000lbs of bombs. A maximum load of 10,000 pounds could be carried over shorter distances.

In order to meet the specification the Boeing Model 341 design was reworked into the Model 345. The Model 345 was to be a pressurized aircraft with four Wright R-3350 engines replacing the R-2800s of the Model 341, a twelve-man crew, a double-wheeled tricycle undercarriage which retracted into the engine nacelles instead of sideways into the wing, four retractable turrets each carrying a pair of 0.5-inch machine guns, and a tail turret with two machine guns and a 20-mm cannon. The retractable Sperry power turrets were operated under remote control by gunners sighting through periscopes. The Model 345 was to be capable of carrying a ton of bombs over 5000 miles at a cruising speed of 290 mph. The maximum bombload was to be 16,000 pounds. The maximum speed was estimated to be 382 mph at 25,000 feet. The weight was projected to be 97,000 pounds.

The Boeing Model 345 design was submitted to the Army on May 11, 1940. The Army was sufficiently interested that it awarded Boeing an appropriation for wind-tunnel tests on June 17. On June 27, 1940, the Army issued contracts for preliminary engineering data for the new "superbomber" to four manufacturers which were designated in order of preference as Boeing XB-29, Lockheed XB-30, Douglas XB-31, and Consolidated XB-32. Lockheed and Douglas both withdrew from the competition before any detailed designs could be completed. On August 24, 1940, the Army ordered two prototypes and a static test model from Boeing under the designation XB-29. In addition, two XB-32 prototypes were ordered from Consolidated as insurance against the failure of the Boeing XB-29.

	Consolidated XB-32
	Ordered as a back-up in case the B-29 project failed, the XB-32 was in many ways an "expanded" Liberator with pressurized fuselage. After the first prototype was built, the twin fin configuration was dropped in favor of a sigle large fin. About 15 aircraft were produced for testing, however it was never put into production. The technology served Consolidated well though - some of the fuselage structure was later carried through to their post war B-36 bomber.