The One and Only Boeing XB-15 

VA – Only one Boeing XB-15 bomber prototype was built, yet it gave the giant aircraft manufacturer 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. In addition, 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 the 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 for 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 stood out. 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 the optimum span for the Clipper was computed to be 152 feet.