Aircraft history

In the early 1930s, aviation technology began to accelerate, driven by improved commercial aircraft.

The internal bracing of wings made it possible to say goodbye to the biplane, while the higher landing speed caused by the monoplanes’ smaller wing area was offset by the development of retractable flaps that enlarged the wing area for landing and takeoff, braked wheels and variable pitch propellers. This  greatly improved engine performance at the upper and lower ends of the speed range and all metal stressed skin construction for stronger fuselages.

These aerodynamic advances made it possible for the U.S. Army Air Corps to pursue development of a true strategic bomber with a speed of 200 mph, believing that fighters fast enough to intercept 200 mph bombers would, as an Air Corps General said, “be too difficult to be efficiently or safely operated either individually or in mass.”

The Boeing Company of Seattle, Wash., had been quick off the mark with transport aircraft and, after some initial struggles, moved on to  the truly heavy, long range, four-engine bomber – the Model 294, which was later renamed the “Experimental Bomber, Long Range” XBLR-1.

On June 29, 1935, a contract was approved for one example of the XBLR-1, and the aircraft was redesignated XB-15.
At about the same time, Boeing proposed another small four-engine bomber, the Model 299, a scaled down XB-15 with a smaller wing and an aircraft that did not “push the design envelope” quite as far.

The XB-15 was an all-metal four-engined mid-wing monoplane that was huge – a
wingspan of almost 150 feet and a fuselage of almost 90 feet long – and powered by four Allison V-1710 liquid cooled V-12 engines, though before the aircraft was built the engines were changed to four 1,000 air-cooled radials.

It was expected to have a range of more than 5,000 miles, so it had previously unheard of luxuries for the crew – a soundproof, heated and ventilated crew section with bunk beds, a kitchenette and a toilet. In an aviation first, it had the first small auxiliary engines to power a 110-volt electrical system, and all engines could be serviced from a passageway inside the thick wing behind the nacelles.
It also carried the incredibly heavy (for the time) defensive armament six machine guns, three .30-caliber and three .50-caliber, including one .50-caliber gun in a manually operated top turret, as well as a bomb load of 2,000 pounds.

Unfortunately, the great size of the XB-15 made its development time very long, and by the time it flew for the first time on Oct. 15, 1937, the older Model 299 had been flying for two years and a number had already been ordered by the Army Air Corps.

This was fortunate because test flights on the XB-15 showed it was underpowered and, compared to the Model 299 t, was much slower, had a lower service ceiling and a smaller bomb load, though it did have much longer range.

Two later models of the XB-15 were ordered with 1,400 horsepower Pratt & Whitney R-2180 radials, but they were canceled and the XB-15 was never ordered into production.
The prototype was turned over to the 2nd Bomb Group in August 1938 and did manage to set several world records for weight carrying, including a 71,167-pound payload lifted to 8,200 feet on July 30, 1939, and a payload of 4,409 pounds carried over a distance of 3,107 miles at 166 mph.
Still, for a “one off” experimental military aircraft, the XB-15 had remarkable longevity and performed useful operational service for eight years, beginning when carried emergency 3,250 pounds of medical supplies to Chile in January 1939, a 4,933-mile trip that took 29 hours and 53 minutes.

During World War II, it was redesignated XC-105 and transferred to the new Air Transport Command at Miami Army Air Field, Fla., where it carried cargo and military personnel to the Panama Canal Zone during the war and was nicknamed “Old Grandpappy.”

Boeing took the design of the wings and engine nacelles of the XB-15, mounted four new 1,500 engines to eliminate the lack of power that handicapped the bomber, and added the wing to a flying boat fuselage to make the wildly successful “China Clipper” that set the standard for long range commercial aviation across the Pacific. The Model 299, of course, went on to become the B-17 Flying Fortress.

For questions or comments, contact Dr. Michel at marshall.michel@ramstein.af.mil.