Boeing XB-15 (XBLR-1 / Grandpappy) Long Range Heavy Bomber Prototype
First flying in 1937, the Boeing XB-15 became the largest American-constructed bomber to that point.
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In 1933, the United States Army Air Corps (USAAC) delivered a new specification for a heavy bomber with an inherent operational range of 5,000 miles and capable of a maximum speed of 200 miles per hour with a 2,000lb internal bomb load. The charge fell to both Boeing and Martin to which design work began in 1934. Boeing delivered its submission as the "XB-15" (Boeing Model 294) while Martin followed with the XB-16. The USAAC agreed to further the Boeing initiative and first flight of the prototype was achieved on October 15th, 1937 (the Martin design was never constructed, deemed too slow for the US Army requirement). However, the intended Allison V-3420 liquid-cooled inline piston engines of 1,000 horsepower were not yet ready and 4 x Pratt & Whitney R-1830-11 Twin Wasp radials of 850 horsepower were substituted instead. The original selection of inline engines was unique for many American aircraft of the period relied on air-cooled radial types.
The XB-15 was certainly a design ahead of its time which included its immense size - the largest aircraft ever built in the United States to that point. The design incorporated such features as an autopilot system to alleviate pilot workload on long flights, auxiliary power units as fail safes and wing deicers to protect against environmental factors during take-off. Internally, the wings were so voluminous that man-sized ducts were constructed that allowed technicians access to key components of the wings with the aircraft in flight. Due to the mission endurance required of the crew, bunk beds, a galley and a bathroom were all part of the internal arrangement of the XB-15. The complete crew complement numbered no fewer than ten personnel. The use of four engines also provided for an impressively wide-spanning wing assembly which gave the XB-15 its very identifiable top down profile.
Outwardly, the XB-15 was beginning to set the mold for more famous Boeing bomber designs to follow including the Boeing B-17 "Flying Fortress" and B-29 "Superfortress" of World War 2. The fuselage consisted of a tubular shape with the stepped flight deck fitted well-forward in the design, the nose capped by a "green house" style housing. The fuselage then tapered towards the rear to give the assembly its "tear drop" shape when viewed in the side profile. Wings were fitted high along the fuselage sides with each leading edge housing a pair of radial piston engines through forward-extended nacelles. These nacelles provided excellent views of the engines from each cockpit position. There proved glazed blisters along the aft sides of the fuselage for waist gunners or mission observers. The tail lacked a gun position as seen in the later B-17 and managed a single rounded vertical tail fin with a pair of low-mounted tail planes. The undercarriage consisted of a pair of two-wheeled main landing gear legs and a tail wheel which gave the design its pronounced "nose-up" appearance when at rest - a design arrangement to be utilized in future Boeing propeller-driven bombers.