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Boeing XB-15 (XBLR-1 / Grandpappy)

1937 WORLD WAR 2 X-PLANE

History

In 1933, the United States Army Air Corps (USAAC) proposed a new specification for a heavy bomber with an inherent range of 5,000 miles, a top speed of 200 mph, and an internal bomb load of 2,000 pounds. Both Boeing and Martin, whose design work began in 1934, have been charged. Boeing delivered its template as the "XB-15" (Boeing 294), and Martin subsequently delivered the XB-16. USAAC agreed to move forward with Boeing's plan, and the first flight of the prototype took place on October 15, 1937 (Martin's design was never built because it was deemed too slow for the US Army's requirements).

However, the expected liquid-cooled 1,000-horsepower Allison V-3420 inline-piston engine was not ready, and in its place were 4 x Pratt & Whitney R-1830-11 Twin Wasp Radials rated at 850 horsepower. The original inline engine choice was unique to many American aircraft of the time, which relied on air-cooled radial types.

The XB-15 was certainly ahead of its time, including its sheer sizethe largest American-built aircraft to date. The design includes features such as an autopilot system to reduce pilot workload on long flights, auxiliary power units for failover and wing de-icers to prevent environmental impacts during takeoff. Internally, the wings are so bulky that human-height ducts have been built to give technicians access to critical components of the wing while the plane is in flight. Due to the mission endurance requirements of the crew, bunk beds, kitchen and bathroom are part of the XB-15's interior layout. The complete crew number is not less than ten people.

The use of four engines also provides an impressive long-span airfoil configuration, which gives the XB-15 a very recognizable top-to-bottom profile.

Externally, the XB-15 began to provide shapes for subsequent, more famous Boeing bomber designs, including the WWII-era Boeing B-17 "Flying Fortress" and B-29 "Superfortress" tubular shapes with stepped flight decks, very advanced In design, the nose is covered by a "Green House" style shell. The fuselage then tapers back and the assembly has a "teardrop shape" when viewed from the side. The wings were mounted high along the sides of the fuselage, and each leading edge housed a pair of radial piston engines through a forward-extending nacelle. These cabins provide excellent views of the engine from any cockpit position. Glass airbags for waist gunners or mission observers were found on the rear side of the fuselage.

As seen on later B-17s, the tail had no gun positions and managed to have a round vertical fin with two low-mounted fins. The landing gear consists of a pair of two-wheeled main gear legs and a tail wheel, which gives the design a distinctive "nose-up" appearance at rest - an arrangement intended for use on future Boeing propeller bombers.

The XB-15 was delivered to the 2nd Bomb Group at Langley Field, Virginia, after the 1939 Chile earthquake and forced into humanitarian service. The aircraft then began formal bomb testing in Panama in April.

During testing, accuracy proved elusive, with less than 1 percent of the ammunition actually hitting the target. The XB-15 crew subsequently set several payload altitude recordson July 30, 1939, the XB-15 recorded a 31,205-pound flight at 8,200 feet.

Officially (according to Boeing sources), the XB-15 has been approved to carry up to 8,000 pounds of ammunition. Her final militarized form also included a defensive network of 3 x .30 caliber machine guns and 3 x .50 caliber machine guns, one of which was mounted on her glass nose.

The machine guns were removed as the plane arrived at Duncan Field in Florida in 1940.

With the United States entering the war following the Japanese attack on Pearl Harbor in December 1941, the XB-15's future became limited as technological advances surpassed its once evolutionary status. The B-17 was firmly positioned as the USAF's primary bomber (as was the Joint B-24 Liberator), while the B-29 was close to combat status - the latter would soon be a technological marvel of its own by the end of the war.

It was decided to convert the existing XB-15 airframe into a dedicated cargo transport aircraft, and on 6 May 1943 the aircraft was redesignated as an experimental "XC-105" classification. The fuselage is fitted with a pair of cargo doors and an internal lift system, making the XC-105 the equivalent of an Air Force mule.

The aircraft remained in service in this capacity until it was officially retired by the USAAF (United States Army Air Force) on December 18, 1944. With the end of the European War in May 1945, the XB-15/XC-105 airframe was dismantled and scrapped at Albrook Field, Panama. Her grenade was thrown in the swamp near the airport, where it sank.

During its active tenure, the XB-15 served only two US groups - the aforementioned USAAC 2nd Bomb Group and USAAF's 20th Force Carrier Squadron.

Work on the XB-15 influenced the planned private program of Boeing Y1B-20 heavy bombers, two of which were ordered by the U.S. Army in 1938. However, the order was quickly cancelled before the physical work began. Nonetheless, Boeing's work on such heavy military bombers provided the U.S.

Air Force with expertise in the design, development, and production of several key operational "heavy aircraft" during World War II and beyondeven for Boeing's commercial involvement Airliner Airliner flying provides expertise.