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Boeing YB-40 Flying Fortress

Bomber Escort Prototype Aircraft

United States | 1943

"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: 08/07/2018 | Content ©www.MilitaryFactory.com | The following text is exclusive to this site; No A.I. was used in the generation of this content.
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.

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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.

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Power & Performance
Those special qualities that separate one aircraft design from another. Performance specifications presented assume optimal operating conditions for the Boeing YB-40 Flying Fortress Bomber Escort Prototype Aircraft.
4 x Wright R-1820-65 turbosupercharged radial piston engines developing 1,200 horsepower each.
292 mph
470 kph | 254 kts
Max Speed
29,199 ft
8,900 m | 6 miles
Service Ceiling
2,262 miles
3,640 km | 1,965 nm
Operational Range
City-to-City Ranges
Operational range when compared to distances between major cities (in KM).
The nose-to-tail, wingtip-to-wingtip physical qualities of the Boeing YB-40 Flying Fortress Bomber Escort Prototype Aircraft.
73.8 ft
22.50 m
O/A Length
103.3 ft
(31.50 m)
O/A Width
19.0 ft
(5.80 m)
O/A Height
55,116 lb
(25,000 kg)
Empty Weight
73,998 lb
(33,565 kg)
Design Balance
The three qualities reflected below are altitude, speed, and range. The more full the box, the more balanced the design.
Available supported armament and special-mission equipment featured in the design of the Boeing YB-40 Flying Fortress Bomber Escort Prototype Aircraft .
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.
Notable series variants as part of the Boeing YB-40 Flying Fortress family line.
XB-40 - Prototype model deisgnation
YB-40 - Base Series Designation; 25 examples completed.
TB-40 - YB-40 gun bus aircraft reworked as trainers.
Global customers who have evaluated and/or operated the Boeing YB-40 Flying Fortress. Nations are displayed by flag, each linked to their respective national aircraft listing.

Total Production: 25 Units

Contractor(s): Boeing Company - USA
National flag of the United States

[ United States (limited) ]
Relative Max Speed
Hi: 300mph
Lo: 150mph
Aircraft Max Listed Speed (292mph).

Graph Average of 225 MPH.
Era Crossover
Pie graph section
Showcasing Aircraft Era Crossover (if any)
Max Alt Visualization
Small airplane graphic
Production Comparison
Entry compared against Ilyushin IL-2 (military) and Cessna 172 (civilian) total production.
MACH Regime (Sonic)
RANGES (MPH) Subsonic: <614mph | Transonic: 614-921 | Supersonic: 921-3836 | Hypersonic: 3836-7673 | Hi-Hypersonic: 7673-19180 | Reentry: >19030
Aviation Timeline
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Image of the Boeing YB-40 Flying Fortress
Image from the Public Domain.
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Image of the Boeing YB-40 Flying Fortress
Image from the Public Domain.

Mission Roles
Some designs are single-minded in their approach while others offer a more versatile solution to airborne requirements.
Some designs stand the test of time while others are doomed to never advance beyond the drawing board; let history be their judge.
Going Further...
The Boeing YB-40 Flying Fortress Bomber Escort Prototype Aircraft appears in the following collections:
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