The XB-15 was a mammoth presence that advanced the state of the art even though it did not rate production status. (Alan Griffith via Gerald Balzer collection)

Only one Boeing XB-15 bomber prototype was built, yet it provided the giant aircraft manufacturer with design ideas that shaped the famous B-17 and Model 314 Clipper airplanes.

The Boeing XB-15 (Model 294), originally known by the nomenclature XBLR-1, or Experimental Bomber, Long Range, Design 1, was conceived before the B-17 Flying Fortress, yet first flew after the original Model 299 that inaugurated the B-17 series.

The XB-15 met an Air Corps requirement for a bomber with a 5,000-mile range, however, it suffered from a common malady of the time. Its airframe was so large, four engines could only give it modest performance.

Originally intended to be powered by four Allison V-1710 liquid-cooled engines, the XB-15 switched to radial Pratt and Whitney R-1830s before it flew.

The XB-15 pioneered Boeing’s mid-1930s bomber aesthetic, with circular fuselage cross-section, broad chord wings, tailwheel stance, and Plexiglas glazed blisters for a host of hand-held machine guns.

When the Army put out a call for a multi-engine bomber that led to the creation of the B-17, Boeing engineers embarked on a quest for a slimmed-down airframe that gave the B-17’s four engines less to lug around, thereby enhancing performance and creating a viable combat aircraft for World War II.

The larger XB-15 was forever saddled with a porkier silhouette.

The XB-15 also relied on dual-wheeled main landing gear sets, borrowing something as old as the gear style of World War I bombers, to spread the load of its 35-ton gross weight.

Photos of the XB-15’s ample interior wing space show provision for additional bomb bays in each wing root, along with the conventional fuselage bomb bay.

Designers at Boeing also equipped the XB-15 with galley and bunk facilities since the big bomber could stay aloft as much as 24 hours, and relief crews were called for.

The XB-15 had another gift for Boeing designers.

Wellwood Beall was a young, bright engineer who lobbied for Boeing to submit a bid in response to Pan American Airways’ need for a new airliner seaplane design in 1936. Initially working at home at night on the concept he wanted Boeing to complete, Beall kept dated and signed sketches and notes on his work.

One notation stands out in particular. Beall employed the massive wing design from the XB-15 for his Clipper concept, saving a lot of design time and effort. But the bomber prototype had a wingspan of 149 feet, and optimum span for the Clipper was computed to be 152 feet.

Beall’s solution was penciled in a memo: “Increase span by adding 36 inches to body width.” The Clipper’s spacious hull was due, at least in part, to this practical adaptation of the XB-15’s wing.

The lone XB-15 was an artifact of its time.

The Air Corps, jockeying for significant missions in the defense rationale of the pre-war United States, put a lot of faith in bombardment aviation. Pursuit aviation — fighters — had advocates in the service as well. But transport and cargo missions were almost afterthoughts during Depression-era budgets. Air Corps exercises with ground army units sometimes employed bombers as carriers of equipment and supplies for the soldiers, because dedicated airlift was not yet the global game-changer it would become during World War II.

The XB-15 spanned 149 feet. Its length was 87 feet, 7 inches. Empty weight was 37,709 pounds. Cruising speed is listed as 152 miles per hour at 6,000 feet. It was said to eke out barely 200 miles per hour at only 5,000 feet in altitude. Range is listed at 5,130 miles.

The XB-15’s performance indicated it was not the answer for heavy bombardment aviation. It could not attain speeds considered necessary for a bomber.

But the budget-conscious Air Corps was not about to toss the big prototype airplane on the scrap heap.

Assigned to the 2nd Bomb Group at Langley Field, Virginia, in 1938, the XB-15 engaged in flights that showcased its endurance and load-carrying capabilities.

It was an era when production lines still used a lot of hand-fitting of parts, so the notion of keeping a single prototype airplane in operational service thanks to the craftsmanship and ingenuity of the mechanics was less daunting than it might appear in today’s world.

When an earthquake ravaged Chile in 1939, the XB-15 was dispatched from Virginia on Feb. 4 with 3,250 pounds of medical supplies from the Red Cross. That flight showcased the B-15’s range and load capabilities, and earned its crew the Mackay Trophy for the most important flight of 1939.

Major Caleb V. Haynes was the pilot on this and other news-making flights in the XB-15. By contemporary accounts, Haynes was a large man and the bigger-than-life XB-15 was a good match.

On July 30, 1939, Haynes and copilot Capt. William D. Old eased the XB-15 into the sky from Patterson Field, Ohio, on a record quest. Carrying a then-staggering payload of 15.5 tons, the laboring XB-15 attained a required altitude of 2,000 meters (more than 6,500 feet). The flight bested the previous payload record set by a Soviet pilot three years earlier by more than a ton.

Three days later, Haynes and the XB-15 were at it again, this time establishing a new record over a closed course by carrying 2,000 kilograms — 4,409 pounds — at a speed averaging more than 166 mph on a distance of 5,000 kilometers or 3,107 miles.

With America’s entry into World War II, the XB-15 flew many times as a de facto transport. The aircraft was sent to the San Antonio Air Depot for modifications to enhance its use as a cargo and transport plane. The converted XB-15 bomber was placed in the cargo aircraft category as the XC-105 by mid-1943.

Operating out of Albrook Field, Panama, the XC-105 repeatedly flew freight and passengers to Miami and out to the Galapagos Islands. In this way, it contributed to the war effort by supporting necessary activity in the region to protect the Panama Canal from attack. Payloads of 15 tons were noted.

By December 1944, the XC-105, nicknamed Grandpappy, was retired.

Grandpappy’s 18 months of troop carrier service in the Caribbean included tallying 100,000 pounds of freight, 50,000 pounds of mail, and more than 5,300 passengers — big numbers for a lone pre-war aircraft to achieve.

In May 1945, orders came to scrap the XC-105 in Panama. Bombardment aviation had matured with more capable aircraft like the B-29 and the forthcoming B-36. Transport aviation looked forward to purpose-built cargo aircraft like the C-82, C-54, C-97, and the C-74.

Grandpappy quietly became a footnote to the history of both of those genres of military aviation.