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Boeing XB-15 (XBLR-1 / Grandpappy)

United States (1937)
Picture of 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.


Detailing the development and operational history of the Boeing XB-15 (XBLR-1 / Grandpappy) Long Range Heavy Bomber Prototype.  Entry last updated on 4/25/2016. Authored by Staff Writer. Content ©www.MilitaryFactory.com

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.
The XB-15 was delivered to the 2nd Bombardment Group at Langley Field, Virginia and was pressed into humanitarian service in 1939 following an earthquake in Chile. The aircraft then underwent official bombing tests in Panama beginning in April. During the tests, accuracy proved elusive for less than 1% of dropped ordnance actually hit its mark. Several load-to-altitude records were then set by XB-15 crews in the period following - on July 30th, 1939, the XB-15 recorded a flight carrying a 31,205lb load up to an altitude of 8,200 feet. Officially (according to Boeing sources), the XB-15 was cleared for a combat ordnance load of up to 8,000lbs. Her final militarized form also included a defensive network of 3 x 0.30 caliber machine guns and 3 x 0.50 caliber heavy machine guns including one at her glazed-over nose. In 1940, the machine guns were removed when the aircraft arrived at Duncan Field, Florida.

With the United States entry into the war following the Japanese surprise attack on Pearl Harbor in December of 1941, the XB-15's future became a limited one as technological advancements overtook her once-evolutionary status. The B-17 was firmly entrenched at a primary bomber for the USAAF (as was the Consolidated B-24 Liberator) while the B-29 was nearing operational status - the latter soon to become its own technological marvel by war's end.

It was decided that the existing XB-15 airframe be converted as a dedicated cargo transport and, on May 6th, 1943, the aircraft was redesignated under the experimental "XC-105" classification. The fuselage was given a pair of cargo doors and an internal hoist system was installed to what would make the XC-105 become the air force equivalent of a pack mule. The aircraft served in this role until December 18th, 1944 before seeing her official retirement by the USAAF (United States Army Air Forces). With the end of the war in Europe by May 1945, the airframe of XB-15/XC-105 was ordered stripped and scrapped while based at Albrook Field in Panama. Her shell was dropped in a swamp near the airfield where she sank. During her active tenure, the XB-15 only served two US groups - the aforementioned 2nd Bombardment Group of the USAAC and the 20th Troop Carrier Squadron of the USAAF.

Work on the XB-15 went on to influence the proposed private venture Boeing Y1B-20 heavy bomber design of which two were ordered by the US Army in 1938. However, this order was quickly cancelled before physical work had begun. Nonetheless, Boeing's work on such heavy-class military bomber aircraft provided the know-how in the design, development and production of several key operational "heavies" for United States air power during World War 2 and beyond - even leading up to Boeing's involvement in commercial passenger flight.






Our Data Modules allow for quick visual reference when comparing a single entry against contemporary designs. Areas covered include general ratings, speed assessments, and relative ranges based on distances between major cities.

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Relative Maximum Speed Rating
Hi: 200mph
Lo: 100mph
    This entry's maximum listed speed (197mph).

    Graph average of 150 miles-per-hour.
City-to-City Ranges
NYC
 
  LON
LON
 
  PAR
PAR
 
  BER
BER
 
  MOS
MOS
 
  TOK
TOK
 
  SYD
SYD
 
  LAX
LAX
 
  NYC
Graph showcases the Boeing XB-15's operational range (on internal fuel) when compared to distances between major cities.
Aviation Era
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Unit Production Comparison
Comm. Market HI*: 44,000 units
Military Market HI**: 36,183 units
1
1


  * Commercial Market High belongs to Cessna 172.

  ** Military Market High belongs to Ilyushin Il-2.


Altitude Visualization
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Origin: United States
Year: 1937
Type: Long Range Heavy Bomber Prototype
Manufacturer(s): Boeing Company - USA
Production: 1
Global Operators:
United States
Historical Commitments / Honors:

Military lapel ribbon for Operation Allied Force
Military lapel ribbon for the Arab-Israeli War
Military lapel ribbon for the Battle of Britain
Military lapel ribbon for the Battle of Midway
Military lapel ribbon for the Berlin Airlift
Military lapel ribbon for the Chaco War
Military lapel ribbon for the Cold War
Military lapel ribbon for the Cuban Missile Crisis
Military lapel ribbon for the Falklands War
Military lapel ribbon for the French-Indochina War
Military lapel ribbon for the Golden Age of Flight
Military lapel ribbon for the 1991 Gulf War
Military lapel ribbon for the Indo-Pak Wars
Military lapel ribbon for the Iran-Iraq War
Military lapel ribbon for the Korean War
Military lapel ribbon for the 1982 Lebanon War
Military lapel ribbon for the Malayan Emergency
Military lapel ribbon for the attack on Pearl Harbor
Military lapel ribbon for the Six Day War
Military lapel ribbon for the Soviet-Afghan War
Military lapel ribbon for the Spanish Civil War
Military lapel ribbon for the Suez Crisis
Military lapel ribbon for the Vietnam War
Military lapel ribbon for Warsaw Pact of the Cold War-era
Military lapel ribbon for the WASP (WW2)
Military lapel ribbon for the World War 1
Military lapel ribbon for the World War 2
Military lapel ribbon for the Yom Kippur
* Ribbons not necessarily indicative of actual historical campaign ribbons. Ribbons are clickable to their respective campaigns/operations.
Measurements and Weights icon
Structural - Crew, Dimensions, and Weights:
Unless otherwise noted the presented statistics below pertain to the Boeing XB-15 model. Common measurements, and their respective conversions, are shown when possible.

Operational
CREW


Personnel
10


Dimension
LENGTH


Feet
87.60 ft


Meters
26.7 m


Dimension
WIDTH


Feet
149.05 ft


Meters
45.43 m


Dimension
HEIGHT


Feet
18.08 ft


Meters
5.51 m


Weight
EMPTY


Pounds
37,787 lb


Kilograms
17,140 kg


Weight
LOADED


Pounds
70,857 lb


Kilograms
32,140 kg

Engine icon
Installed Power - Standard Day Performance:
4 x Pratt & Whitney R-1830-11 14-cylinder radial piston engines developing 850 horsepower each.

Performance
SPEED


Miles-per-Hour
197 mph


Kilometers-per-Hour
317 kph


Knots
171 kts


Performance
RANGE


Miles
5,133 mi


Kilometers
8,260 km


Nautical Miles
4,460 nm


Performance
CEILING


Feet
18,898 ft


Meters
5,760 m


Miles
3.58 mi

Supported Weapon Systems:

Graphical image of an aircraft medium machine gun
Graphical image of an aircraft heavy machine gun
Armament - Hardpoints (0):

STANDARD:
3 x 0.50 caliber Browning M2 heavy machine guns
3 x 0.30 caliber Browning M1919 machine guns

OPTIONAL:
Up to 8,000-12,000lbs of internal stores.
Variants: Series Model Variants
• XB-15 ("Grandpappy") - Base Series Designation
• XBLR-1 ("Experimental Bomber, Long-Range") - Limited-use designation.
• Model 294 - Boeing Model Number
• XC-105 - Experimental cargo transport conversion of original XB-15 airframe used operationally during World War 2.