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Boeing YB-40 Flying Fortress

Bomber Escort Prototype Aircraft

The YB-40 Flying Fortress was a wartime conversion of the classic B-17 bomber to fulfill a flying gunship role - 25 were built.
Authored By: Staff Writer | Last Edited: 8/7/2018
Like other classic aircraft of the World War 2 period (1939-1945), the Boeing B-17 "Flying Fortress" heavy bomber was subject to many experiments and conversion projects during its time in service. The YB-40 was developed as one of the former and intended to showcased the B-17 as a sort of flying "gun bus" to defend bomber formations to-and-from enemy targets. The project was not an outright success but did yield some twenty-five examples before the end and influence several key changes of the B-17 line going forward.

Prior to the availability of long-range fighter escorts like the North American P-51 Mustang and Republic P-47 Thunderbolt, thought was given to outfitting bomber airframes with additional machine guns, suitable ammunition stocks, and improved armor protection to help them serve as formation escorts. These aircraft were designed to accompany large American bomber formations over enemy airspace, provide point defense against intercepting enemy fighters, and return with the formation once the war load was dropped on enemy targets below. The B-17 seemed a good a starting point as any for it was a proven player and available in the numbers required.

Project V-139 marked the first example of such a creation and this involved a production-quality B-17F model. Lockheed (Vega) handled the early conversion work which involved addition of machine guns at various key locations and armor protection at the gunner's positions. The single guns at each open-air beam position (the openings staggered for better gunner movement in the heat-of-battle) was doubled and a second dorsal turret was added aft of the first. A twin-gunned Bendix powered turret was installed at the chin position. The bombing equipment and bomb bay were deleted and, in the latter's place, a reserve for additional stocks of ammunition was added. The Sperry ball turret under the belly was retained and the cheek machine guns, at the sides of the forward fuselage, were still in play (though not initially installed as part of the conversion work).

The aircraft could carry as many as thirty heavy machine guns if pressed though between fourteen and eighteen proved typical due to the weight gains and practicality. With each gun installation there was an increase in ammunition required and a standard load of nearly 11,000 rounds was seen - most of this dedicated to the two dorsal turrets and nose guns.

Douglas Aircraft was charged with making the modifications for the operational aircraft and, beyond the mentioned gun installations, other arrangements were trialed during this period - some involving automatic cannons of various calibers - though none made it to service.

The product was already on order back in October 1942 when a prototype "XB-40" made a first-flight on November 10th of that year. The USAAF required thirteen converted bombers through an initial batch and a further twelve were added in January of 1943. First production-quality aircraft emerged from Douglas that March and the series carried the developmental designation of "YB-40" for the interim, pending the outcome of their in-service performance.

The aircraft were delivered to England in May of 1943 (one was lost en route, crash-landing in Scotland) and these flew operational-level missions for the USAAF from that point until July of that year. Results were not wholly impressive as the fleet claimed only five enemy fighters from the forty-eight missions flown. The added weight of the armor plate and installed armament meant that these defenders could not keep pace with the main bomber force once the bombers had dropped their war loads. The test program was ended with the last mission flown on July 29th, 1943, this an attack involving a pair of XB-40s on the Kiel submarine pen.

Once their operational usefulness had concluded, the stock was sent back stateside and took part in crew training as the "TB-40". All were scrapped before the end of the war in 1945. The XB-40 was not a total loss as it introduced several key features of future B-17 generations - namely the Bendix chin turret (which proved vital in defending against oncoming attacks from the front) and the staggered beam gun positions. Work on the XB-40 also resulted in upgrades to the tail gunner's position in the way of improved vision for better tracking and engagement of fast-moving targets.

As completed, the YB-40 could boast a maximum speed of 292 mph with a cruise speed of 195 mph. Range was out to 2,260 miles with a service ceiling of 29,200 feet. Power was from 4 x Wright R-1820-65 turbosupercharged air-cooled radial piston engines developing 1,200 horsepower each.


Retired, Out-of-Service
[ 25 Units ] :
Boeing Company - USA
National flag of United States United States (limited)
- X-Plane / Developmental
73.82 ft (22.5 m)
103.35 ft (31.5 m)
19.03 ft (5.8 m)
(Showcased structural dimension values pertain to the Boeing YB-40 Flying Fortress production model)
Empty Weight:
55,116 lb (25,000 kg)
73,998 lb (33,565 kg)
(Diff: +18,883lb)
(Showcased weight values pertain to the Boeing YB-40 Flying Fortress production model)
4 x Wright R-1820-65 turbosupercharged radial piston engines developing 1,200 horsepower each.
(Showcased powerplant information pertains to the Boeing YB-40 Flying Fortress production model)
Maximum Speed:
292 mph (470 kph; 254 kts)
Service Ceiling:
29,199 feet (8,900 m; 5.53 miles)
Maximum Range:
2,262 miles (3,640 km; 1,965 nm)
(Showcased performance values pertain to the Boeing YB-40 Flying Fortress production model; Compare this aircraft entry against any other in our database)
2 x 0.50 caliber Browning M2 heavy machine guns in powered Bendix chin turret.
1 x 0.50 caliber Browning M2 heavy machine gun in left cheek position.
1 x 0.50 caliber Browning M2 heavy machine gun in right cheek position.
2 x 0.50 caliber Browning M2 heavy machine guns in forward dorsal turret.
2 x 0.50 caliber Browning M2 heavy machine guns in aft dorsal turret.
2 x 0.50 caliber Browning M2 heavy machine guns in left beam position.
2 x 0.50 caliber Browning M2 heavy machine guns in right beam position.
2 x 0.50 caliber Browning M2 heavy machine guns in ventral Sperry ball turret.
2 x 0.50 caliber Browning M2 heavy machine guns in tail turret position.
(Showcased armament details pertain to the Boeing YB-40 Flying Fortress production model)
XB-40 - Prototype model deisgnation
YB-40 - Base Series Designation; 25 examples completed.
TB-40 - YB-40 gun bus aircraft reworked as trainers.

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