In April 14, 1934, the Army Air Corps issued a request for a long-range bomber. A 5000 mile range with a 2000-pound bombload was envisaged.
The Boeing Airplane Company submitted its Model 294 in response to this requirement. The Army expressed sufficient interest in the Model 294 that it issued a contract on June 28, 1934 for design data, wind-tunnel tests, and the construction of a mockup under the designation XBLR-1, the letters standing for "Experimental Bomber, Long Range". On June 29, 1935, a contract was approved for one example of the XBLR-1. The BLR category was later eliminated, and the aircraft was redesignated XB-15 in July 1936.
The XB-15 was a large, four-engined mid-wing cantilever monoplane with all-metal semi-monocoque construction. The structure was generally similar to that of earlier Boeing monoplanes that had been based on the Monomail design, with the exception that the wing from the main spar aft was covered with fabric instead of metal. The XB-15 was originally to have been powered by four Allison V-1710 liquid cooled V-12 engines. However, before the aircraft was built, the powerplants were changed to four 1000 hp Pratt & Whitney R-1830 twin-row air-cooled radials.
The crew of ten has soundproofed, heated, and ventilated quarters with rest bunks, a kitchen, and a lavatory. For the first time in an airplane, small auxiliary engines were fitted which powered a 110-volt electrical system. The wing was so thick at the root that it was possible for a crew member to service the engine accessory sections in flight from a passageway extending behind the nacelles. Such a system later appeared on the Boeing Model 314 flying boat. The main undercarriage retracted into the rear of the inner engine nacelle. The aircraft was sufficiently heavy that it was necessary to fit two wheels on each main undercarriage truck.
The defensive armament was the heaviest yet to be fitted to a bomber. It carried six machine guns. One of these guns was mounted in a nose turret and another one was carried in a forward-facing belly turret mounted below the pilot's cabin. A 0.50-inch gun was mounted in a top turret which could rotate through 360 degrees. One gun was carried in each of two waist blisters attached to the fuselage behind the wings. A sixth gun was housed inside a rearward-facing belly turret.
Although it was an older design than the Model 299 which eventually emerged as the B-17 and carried an earlier military designation, the XB-15 made its first flight two years later. The XB-15 (serial number 35-277) made its first flight on October 15, 1937 with Eddie Allen at the controls. At the time of its appearance, it was the largest and heaviest aircraft yet to be built in the United States.
The XB-15 proved to be seriously underpowered with the R-1830 radials. Because of its low performance as compared to later aircraft (the top speed was only 197 mph), the XB-15 was never ordered into production, and the prototype was the only example to be built.
Following the completion of the tests, the XB-15 was turned over to the 2nd Bomb Group in August of 1938. In spite of the fact that the aircraft was seriously underpowered for its size, it did manage to set several world records for weight carrying, including a 71,167-pound payload lifted to 8200 feet on July 30, 1939 and a payload of 4409 pounds carried over a distance of 3107 miles at 166 mph.
In 1943, the sole XB-15 was converted into a cargo carrier by adding cargo doors and a hoist. The aircraft was redesignated XC-105. The gross weight increased to 92,000 pounds. the plane served for 8 years and even carried out antisubmarine patrols. The XC-105 was scrapped at Kelly Field, Texas shortly before the end of the war.
Two service test models of the XB-15 were ordered under the designation Y1B-20. In an attempt to provide more power, these planes were to have been powered by 1400 hp Pratt & Whitney R-2180 radials. However, they were cancelled before anything could be built.
Specification of XB-15:
Four Pratt & Whitney R-1830-11 Twin Wasp air cooled radials, rated at 850 hp at 2450 rpm at 5000 feet, and 1000 hp for takeoff. Maximum speed: 200 mph at 5000 feet, 197 mph at 6000 feet, cruising speed 152 mph at 60 percent power at 6000 feet. Service ceiling 18,900 feet, absolute ceiling 20,900 feet. Climb to 5000 feet in 7.1 minutes, climb to 10,0000 feet in 14.9 minutes. Range 3400 miles with 2511 pounds of bombs, maximum range 5130 miles. Weights: 37,309 pounds empty, 65,068 pounds gross, 70,706 pounds maximum. Dimensions: wingspan 149 feet, length 87 feet 7 inches, height 18 feet 1 inches, wing area 2780 square feet. Armed with two 0.50-inch and four 0.30-inch machine guns. A bomb load of four 2000 pound bombs could be carried. Maximum bombload was 12,000 pounds.