A Case of Gigantism. The Boeing XB-15 shown here was the largest airplane in the world when it was completed in 1937. Although an important step in large bomber development, the plane was slow and underpowered.
With a wingspan of 149ft and a maximum take-off weight of 70,706lb, the Boeing XB-15 was an intimidating looking bomber for the mid-1930s which was let down by a serious lack of horsepower. The sole aircraft nicknamed `Grandpappy’, served with the 2nd BG at Langley Field and later, after being re-designated as the XC-105 transport, with the 20th TCS (Troop Carrier Squadron). The giant aircraft was retired on December 18, 1944.
The XB-15 set a number of `payload to height and speed/weight over distance’ records including the international 5,000km speed record with a 2,000kg payload.
Before the B-17 appeared on the scene, the Air Corps had stated a requirement for a bomber far advanced beyond the Flying Fortress. Known as Project A, this plane was to have a maximum range of 5,000 miles and speed of 200 miles per hour, with a 2,000-pound bomb load. It was to have the ability “to reinforce Hawaii, Panama, and Alaska without the use of intermediate servicing facilities.” In May 1934 the General Staff approved as its tactical mission “the destruction by bombs of distant land or naval targets.” Boeing began development of the plane in June 1934, and the Air Corps contracted for one Project A plane. The Army was interested enough in the Boeing proposal to issue a contract on 28 June 1934 that covered the design, wind-tunnel testing and a mock-up, all covered by the designation XBLR-1(Experimental Bomber Long Range 1). The aircraft passed the design and inspection process successfully and on 29 June 1935 a contract for a single XBLR-1 was issued; the designation was changed to XB-15 in July 1936, when the more long-winded designations were finally dropped from the Army vocabulary.
The aircraft that Boeing produced was a big, four-engined, mid-wing cantilever monoplane of all-metal semi-monocoque construction. Although it was not immediately obvious, the structural design was firmly based around that developed through the Monomail and YB-9 aircraft, the only exception to that rule being that the wing surface area aft of the main spar was covered in fabric instead of metal alloy sheet. [t had originally been intended to install four Allison V-1710 liquid-cooled V-12 engines, but before the design was finally confirmed the Allison engine was cancelled and the XB-15’s powerplant changed to the Pratt & Whitney R- 1830 air-cooled radial. The wings that the engines were mounted on were of sufficient depth to allow the aircraft engineer to enter the space aft of the engines to carry out maintenance in flight, should it be needed – this feature was later covered over to the Boeing Model 314 flying boat.
The ten-man crew was housed in a fuselage that featured heating, pressurization and ventilation as well as crew rest bunks, an in-flight kitchen and a lavatory. The aircraft had a retractable main undercarriage units and tail wheel, the former having two main wheels per leg to reduce the pavement loading factor. The intended defensive armament was the heaviest yet fitted to a combat aircraft consisting of six machine guns. These were located in the nose turret, the forward-facing ventral turret, in the top turret, one in each waist blister and in a rear-facing ventral turret. The basic bomb load was set at 8,000lb (1, 600kg), although a maximum overload of 12,000lb (5,400kg) could be carried over a shorter distance.
It was the largest and heaviest airplane built in the United States up to that time: its wingspan was 149 feet versus 104 for the B-17. It was 20 feet longer, with a gross weight was over 70,000 pounds-more than double that of the Fortress. The problem with the plane was its lack of engine power: its four motors, each producing 1,000 horsepower, were not big enough. One Boeing expert summed the XB-15’s problem nicely: “It provided an example of a typical situation, where a promising new design was handicapped by lack of the bigger power plants necessary to develop its full potential.” Only one prototype was built, and it was later converted to a cargo plane.
The plane flew for the first time in 1937, but it was too large for the power plants then available. The B-17 benefited from early work done on the XB-15, and the B-29 later was the offspring of Project A.
In service for eight years, the XB-15 served with two squadrons at Langley Field, Hampton, Virginia, the 49th Bombardment Squadron (Heavy), 2d Bombardment Group (Heavy) between 1938 and 1940, and the 41st Reconnaissance Squadron (Heavy), 2d Bombardment Wing between 1940 and 1942.
February 14, 1939, Maj. Caleb V. Haynes and his crew flew the Boeing XB-15, laden with more than 3,000 pounds of medical supplies, from Langley Field, Virginia, to Chile for the relief of earthquake victims. The flight demonstrated not only U. S. humanitarian-airlift capabilities, but also the range and payload of the new airplane.
This aircraft was then reassigned to the Sixth Air Force in the Caribbean and arrived at the Panama Air Depot (PAD), Albrook Field, Canal Zone, on 23 March 1943.
This aircraft was underpowered and was never placed in production but was used for experimental tests. Because of its cargo-carrying capacity, it was redesignated XC-105-BO on 6 May 6, 1943 and was modified by the PAD. After modification, it was assigned to the 20th Troop Carrier Squadron, Sixth Air Force Service Command, based at Albrook Field in December 1943.
During its eight year in service, the XB-15/XC-105 flew 60 combat missions including antisubmarine warfare patrols and 70 cargo trips carrying 5,200 passengers, 440,000 lbs (199 581 kg) of cargo and 94,000 lbs (42 638 kg) of mail. The XC-105 was placed in storage at the PAD in May 1944 due to structural damage. This one of kind aircraft was ignominiously shoved into the Curundu Swamp, east of Albrook Field, where it slowly sunk into the muck. It remains there to this day.
XC-105 “Grandpappy” in Panama
Technical Specifications (XC-105 [XB-15]) Type: Long- range troop and cargo transport. Manufacturer: Boeing Airplane Co., Seattle, Washington. Total military versions: 1 (AAF). Capacity: Crew of six plus unspecified number of troops or 27,350 lbs. of cargo. Powerplants: Four 1,000-hp Pratt & Whitney R-1830-11 Twin Wasp 14-cylinder air-cooled radial engines driving three- bladed Hamilton Standard variable- pitch metal propellers. Performance: Max. speed 197-mph, cruise 152-mph; ceiling 18,900 ft.; range 5,130 mi. Weights: 37,309 lbs. empty, 65,068 lbs. gross, 70,706 lbs. max. takeoff. Dimensions: Span 149 ft., length 87 ft. 7 in., wing area 2,780 sq. ft.
The Boeing Model 294, originally designated XBLR-1 (experimental bomber, long- range) but re-designated XB-15 in 1936 while still under construction, had been designed to test the “hemispheric defense” concept, which contemplated an aircraft that could deliver 2,000 lbs. of bombs from U. S. territories to targets within a 2,000-mile combat radius. However, due to the limited powerplant availability at that time, the XB-15, when flown in October 1937, proved to be woefully underpowered in terms of speed and rate- of- climb. Despite its shortcomings, the XB-15 gave Boeing valuable experience in the design and construction of the large aircraft it would be producing in the near future (e. g., Model 314, B-29, C-97). In 1943, after installation of an aft- mounted cargo door and an internal hoist, the XB-15 became a transport under the designation XC-105, with takeoff weight upped to 92,000 lbs. In its new role, the aircraft flew cargo, mail, and passengers on routes in the Caribbean until being scrapped at Kelly Field shortly before the war ended in 1945.