History
Background
On April 14, 1934, the US Army Air Corps issued a request for a “hemisphere defender,” a long-range bomber capable of flying 5,000 miles (8,047 km) while carrying a 2,000-pound (910 kg) payload at 200 mph (322 km/h). To meet this requirement, Boeing submitted its Model 294 design (later designated XB-15), while Martin proposed the Model 145-A (designated XB-16). The Army ultimately favored Boeing’s submission, deeming Martin’s proposal insufficient.
On June 28, 1934, Boeing was awarded a contract to conduct wind-tunnel tests, provide design data, and construct a mockup under the designation XBLR-1 (Experimental Bomber, Long Range). A year later, on June 29, 1935, the Army contracted Boeing to build a prototype, which was redesignated XB-15 in July 1936 when the XBLR category was eliminated.
Design and Features
The XB-15 was a massive, four-engine, mid-wing cantilever monoplane with an all-metal semi-monocoque construction. It was powered by four 1,000-horsepower Pratt & Whitney R-1830-11 engines. Boeing developed the XB-15 based on its earlier YB-9 bomber, though the XB-15’s trailing wing surfaces were covered in fabric instead of metal.
The aircraft’s interior reflected the luxury airliner standards of the era. The ten-man crew was housed in a fuselage equipped with heating, ventilation, rest bunks, a lavatory, and an in-flight kitchen. Engineers could access the engines mid-flight via crawlways inside the wings, enabling repairs. It also featured auxiliary engines to power a 110-volt electrical system, an innovation in aviation at the time. Additionally, the XB-15 was outfitted with cutting-edge autopilot and deicing systems.
The XB-15 had retractable landing gear, with each main undercarriage leg fitted with dual wheels to reduce pavement loading. Defensive armament included six machine guns positioned in the nose turret, forward and rear ventral turrets, top turret, and waist blisters. Sources differ on whether its armament comprised two or three .50-caliber heavy machine guns and four or three .30-caliber medium machine guns, with a total ammunition load of approximately 7,200 rounds. The XB-15 could carry an 8,000-pound (3,600 kg) payload under standard conditions, though it could be overloaded to carry up to 12,000 pounds (5,400 kg) over shorter distances.
Testing and Achievements
First flown on October 15, 1937, the XB-15 became the largest and heaviest bomber ever built in the United States at the time. Its serial number was designated 35-277. However, its sheer weight overtaxed the Pratt & Whitney engines, limiting its maximum speed to just 200 mph (320 km/h). This performance shortfall led the Army to opt for Boeing’s parallel development, the B-17 Flying Fortress, as its primary bomber fleet.
After flight testing, the XB-15 was assigned to the 2nd Bomb Group in August 1938 for service evaluation and to familiarize crews with long-range bomber operations. It demonstrated exceptional endurance and payload capacity.
In February 1939, following a devastating earthquake in Chile, the XB-15 delivered 3,250 pounds (1,470 kg) of Red Cross medical supplies, earning its crew the Mackay Trophy for the most significant humanitarian flight of the year. On July 30, 1939, it set two world records, lifting a 22,046-pound (10,000 kg) payload to 8,228 feet (2,508 m) and a 31,164-pound (14,136 kg) payload to 6,562 feet (2,000 m).
In April 1940, the XB-15 conducted classified bombing tests in Panama, assessing canal lock defenses. Despite only three out of 150 bombs having hit the target, the results informed bunker reinforcement strategies. It also undertook reconnaissance missions over the Galápagos Islands. Later in 1940, its defensive armament was removed during maintenance at Duncan Field, Texas.
Wartime Service as XC-105
With the onset of World War II, the XB-15 was converted into a heavy cargo aircraft and redesignated XC-105 in May 1943. Modifications included a cargo hoist, larger cargo doors, and an increased maximum gross weight of 92,000 pounds (42,000 kg). Nicknamed “Grandpappy” by airmen of the 20th Troop Carrier Squadron, the XC-105 was decorated with a nose art featuring an elephant carrying a crate labeled “supplies.”
Operating primarily from Albrook Field in Panama, the XC-105 transported freight, passengers, and mail across the region, including to Miami and the Galápagos Islands. It also conducted antisubmarine patrols to protect the Panama Canal. Over 18 months of service, it carried 100,000 pounds (45,360 kg) of cargo, 50,000 pounds (22,680 kg) of mail, and more than 5,300 passengers—a remarkable achievement for a pre-war aircraft.
After eight years of service, the XC-105 was retired in December 1944. In May-June 1945, it was scrapped at Albrook Field in Panama, just around the end of World War II. The remaining scrapped airframe was left at Diablo dump, a swampy landfill southwest of the runway, where it was sunk out of sight. Today, the former dump is now an industrial area, with “Grandpappy” buried underneath.
