On October 15, 1937 the Boeing XB-15, a groundbreaking aircraft, took its first flight and left an indelible mark on aviation history, despite being a lesser-known bomber prototype. Built in response to a desire for a long-range heavy bomber, the XB-15 was a product of both ambition and experimentation by the United States Army Air Corps (USAAC). Though it ultimately didn’t go into mass production, the aircraft set records, influenced future designs, and played a critical role in humanitarian missions and military operations. The story of the XB-15 begins in 1934, when the USAAC sought to explore the potential of a bomber capable of traveling 5,000 miles while carrying a 2,000-lbs payload at speeds of 200 mph. This would allow for long-range strikes, giving the U.S. an unprecedented strategic capability. The initial project, referred to as Project A, began in mid-1933, with contracts awarded to both Boeing and Martin for design proposals.
The Challenging Design
While Martin’s design, the XB-16, was quickly deemed inferior and canceled, Boeing’s internal Model 294 caught the attention of the USAAC. The aircraft would soon be officially designated as the XB-15. However, it wasn’t always known by that name. In 1935, the aircraft received the temporary designation XBLR-1 (Bomber, Long Range), signifying its role as a long-range bomber experiment. The name XB-15 returned the following year. The XB-15 faced a long development process, largely due to its sheer size and complexity. At the time, it was the largest aircraft ever built in the United States, a behemoth that presented unprecedented engineering challenges. Under the leadership of Jack Kylstra, Boeing’s design team intended to equip the aircraft with 2,600-hp Allison V-3420 liquid-cooled engines. However, these engines were unavailable at the time, so the XB-15 ended up with less powerful 850-hp Pratt & Whitney R-1830 Twin Wasp radial engines. This switch to air-cooled engines significantly underpowered the aircraft, hindering its performance potential. The XB-15’s immense dimensions were impressive with a length of 26.7 meters and a wingspan of 45.4 meters. It was capable of carrying a bomb load of up to 12,000 lbs, the XB-15 also featured an internal bomb bay—an innovation at the time. Defensive armament consisted of three .30-caliber and three .50-caliber machine guns positioned throughout the aircraft. The XB-15’s performance specifications reflected its unique role as a long-range bomber with a maximum speed of 197 mph at 5,000 feet and a cruising speed of 152 mph at 6,000 feet. One of its defining features was its ability to carry engineers inside the wings during flight, allowing them to perform in-flight maintenance, a necessity for such a large, complex machine.
First Flight and Early Performance
On October 15, 1937, the XB-15 took to the skies for the first time, piloted by Edmund Turney “Eddie” Allen, Boeing’s chief test pilot, and Major John D. Corkille from the USAAC. The flight took place at Boeing Field in Seattle, Washington. Remarkably, for such a massive aircraft, the XB-15 handled well, with Major Corkille noting that it “handled easily and maneuvered readily.” The successful maiden flight set the stage for further testing and evaluation. After a successful start, the XB-15 set records early in its operational life. On July 30, 1939, it achieved a significant milestone by lifting a 31,205 lbs payload to an altitude of 8,200 feet. This set the load-to-altitude record for land-based aircraft at the time, solidifying the XB-15’s reputation as an engineering marvel. The aircraft also broke another record in June 1939 when it carried a 22,046 lbs payload to 8,228 feet and a 31,164 lbs payload to 6,561 feet. These record-setting flights demonstrated the bomber’s capabilities despite its underpowered engines, highlighting the value of its large airframe for heavy-lift missions.
Humanitarian and Military Missions
Though only one XB-15 was built, its operational history was unique and varied. It was assigned to the 2nd Bombardment Group at Langley Field, Virginia, where it undertook several missions that went beyond standard bombing tests. One of its most notable missions occurred in January 1939, when the aircraft was deployed to Chile following the Chillán earthquake. Under the command of Major Caleb V. Haynes, the XB-15 delivered 3,250 pounds of American Red Cross supplies to Santiago. This humanitarian mission earned Major Haynes the Distinguished Flying Cross and the Order of the Merit of Chile, while the entire crew received the MacKay Trophy for their efforts. In another significant mission, the XB-15 was tasked with returning the body of Mexican aviator Francisco Sarabia to Mexico City after Sarabia tragically died in a crash. Following this mission, the aircraft participated in classified bombing tests in the Panama Canal Zone in April 1940, where it tested defenses and bunker designs to safeguard the canal from potential attacks.
Final Years
As the XB-15 became obsolete as a bomber due to its underpowered engines, it was converted into a transport aircraft and redesignated XC-105 in May 1943. The conversion included installing a cargo hoist, fitting cargo doors, and increasing the maximum gross weight to 92,000 pounds. Nicknamed “Grandpappy” by its crew, the XC-105 played an essential role in transporting personnel and cargo during World War II, ferrying supplies and passengers across the Caribbean and Florida. Though “Grandpappy” contributed significantly to logistics efforts, the aircraft proved challenging to operate, with its crew reporting fires and electrical system failures. By December 1944, the XC-105 was retired and ultimately scrapped at Albrook Field in Panama in 1945.
Tribunal End
Despite the XB-15’s limited production, its contributions to aviation were substantial. It helped test and prove the feasibility of long-range bombers, directly influencing future aircraft such as the Boeing B-17 Flying Fortress and the B-29 Superfortress. Its innovations in range, payload, and airframe design became foundational for the development of heavy bombers used during and after World War II. The XB-15 also demonstrated the versatility of large aircraft, transitioning from a bomber to a transport and playing crucial roles in both military and humanitarian missions. Its record-breaking achievements and operational legacy solidified its place in aviation history as an aircraft ahead of its time, paving the way for future advances in long-range aviation.
Today in Aviation History is a series highlighting the achievements, innovations, and milestones that have shaped the skies. All the previous anniversaries are available HERE.
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"Haritima Maurya, pen name, ""Another Stardust,"" has been passionate about writing since her school days and later began sharing her work online in 2019. She was drawn to writing because of her love for reading, being starstruck by the art of expression and how someone can make you see and feel things exclusive to their experience. She wanted to be able to do that herself and share her mind with world cause she believes while we co exist in this beautiful world least we can do is share our little worlds within.
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