PlaneSpottingWorld welcomes all new members! Please gives your ideas at the Terminal.

YB-40 Flying Fortress

From PlaneSpottingWorld, for aviation fans everywhere
YB-40 Flying Fortress
Type Escort bomber
Manufacturer Lockheed-Vega
Maiden flight September 1943
Introduced 29 May 1943
Retired October 1943
Status Out of service
Primary user U.S. Army Air Force
Number built 25
Developed from B-17 Flying Fortress
This article is about the military aircraft. For the Vietnamese variant of the RPG-2 see B-40

The Boeing YB-40 Flying Fortress was a modification of the B-17 Flying Fortress bomber aircraft, converted to act as a heavily-armed escort for other bombers during World War II. At the time of its development, long-range fighter aircraft such as the P-51 Mustang were not yet available to accompany bombers all the way from England to Germany and back.


Initial work on the project began in September, 1942 with the XB-40 prototype, built by Lockheed's Vega subsidiary. The first order of 13 was made in October. A follow-up order for 12 more was made in January, 1943. The modifications were performed by Douglas Aircraft at their Tulsa, Oklahoma center, and the first aircraft were completed by the end of March, 1943.

The aircraft differed from the standard B-17 in that a second dorsal turret was installed in the former radio compartment (between the top turret and the waist guns); the single 0.50-calibre (12.7 mm) machine gun at each waist station was replaced by a pair of 0.5-calibre (12.7 mm) guns; and the bombardier's equipment was replaced with two 0.50-calibre (12.7 mm) machine guns in a "chin" turret. The existing "cheek" machine guns, initially removed from the configuration, were restored in England to provide a total of sixteen and the bomb bay itself was converted to a magazine. However a significant portion of the 4,000 pound weight increase came from armor plates installed to protect crew positions. An indication of the burden this placed on the YB-40 is that while the B-17F on which it was based was rated to climb to 20,000 feet in 25 minutes, the YB-40 was rated at 48 minutes.

The YB-40's mission was to provide a heavily-gunned escort capable of accompanying the bombers all the way to the target and back. Overall the concept proved a failure because the YB-40 could not keep up with standard B-17Fs, particularly after they had dropped bombs. Of the initial order of 13, one was damaged in a forced landing on the Isle of Lewis en route to England, and the remaining 12 were assigned to the 92nd Bomb Group (H) and designated the 327th Bomb Squadron.

Between May 29 and August 16, 1943, the YB-40 flew 14 of the 19 combat missions scheduled by the 8th Air Force, although on the mission of June 26 all the YB-40s scheduled were unable to complete assembly and returned to base. Altogether of the 59 aircraft despatched, 48 sorties were credited. 5 kills and 2 probables (likely kills) were claimed on the 13 missions flown, and one YB-40 was lost, shot down by flak on the June 22 mission to Hüls, Germany. Tactics were revised on the final five missions by placing a pair of YB-40's in the lead element of the strike to protect the mission commander.

One YB-40 of the second order, reflecting modifications requested during combat trials to lighten the aircraft, joined the 327th in October, 1943, but by then B-17G models were beginning to appear and the final YB-40 was not flown in combat. All the deployed YB-40s were returned to the United States and converted to training aircraft, as were 11 aircraft of the second order.

Despite the failure of the project as an operational aircraft, it led directly to modifications conspicuous on the final production variant of the B-17, the B-17G: the chin turret, off-set waist gun positions, and a lightweight tail position.

Specifications (YB-40)

General characteristics



Front top turret2500
Aft top turret3300
Ball turret300
Waist guns1200
Tail guns1200

References and external links

  • Freeman, Roger A. (1990). The Mighty Eighth War Diary. ISBN 0-87938-495-6. 
  • Freeman, Roger A. (1991). The Mighty Eighth War Manual, 154-155. ISBN 0-87938-513-8. 
  • Bishop, Cliff T. (1986). Fortresses of the Big Triangle First, 69, 73, 246-247. ISBN 1-869987-00-4. 
  • Encyclopedia of American Aircraft
  • USAF Museum article on YB-40

Related content

Related development

Designation sequence

es:Boeing B40