page_title: Boeing B-17 Flying Fortress
section_title: Design and variants
hierarchical_section_title: Boeing B-17 Flying Fortress / Design and variants
caption_attribution_description: English: Top view of Boeing SB-17G-95DL (S/N 44-83722), assigned to the 2nd ERS as a Search and Rescue aircraft, with Stinson L-5 (S/N 42-98578).
context_page_description: The Boeing B-17 Flying Fortress is a four-engined heavy bomber developed in the 1930s for the United States Army Air Corps. Competing against Douglas and Martin for a contract to build 200 bombers, the Boeing entry outperformed both competitors and exceeded the air corps' performance specifications. Although Boeing lost the contract because the prototype crashed, the air corps ordered 13 more B-17s for further evaluation. From its introduction in 1938, the B-17 Flying Fortress evolved through numerous design advances, becoming the third-most produced bomber of all time, behind the four-engined Consolidated B-24 Liberator and the multirole, twin-engined Junkers Ju 88.
  The B-17 was primarily employed by the USAAF in the daylight strategic bombing campaign of World War II against German industrial and military targets.
context_section_description: The aircraft went through several alterations in each of its design stages and variants. Of the 13 YB-17s ordered for service testing, 12 were used by the 2nd Bomb Group of Langley Field, Virginia, to develop heavy bombing techniques, and the 13th was used for flight testing at the Material Division at Wright Field, Ohio. Experiments on this aircraft led to the use of a quartet of General Electric turbo-superchargers which would become standard on the B-17 line. A 14th aircraft, the YB-17A, originally destined for ground testing only and upgraded with the turbochargers, was redesignated B-17A after testing had finished.
  As the production line developed, Boeing engineers continued to improve upon the basic design. To enhance performance at slower speeds, the B-17B was altered to include larger rudders and flaps. The B-17C changed from three bulged, oval-shaped gun blisters to two flush, oval-shaped gun window openings, and on the lower fuselage, a single "bathtub" gun gondola housing, which resembled the similarly configured and located Bodenlafette/"Bola" ventral defensive emplacement on the German Heinkel He 111P-series medium bomber.
  While models A through D of the B-17 were designed defensively, the large-tailed B-17E was the first model primarily focused on offensive warfare. The B-17E was an extensive revision of the Model 299 design: The fuselage was extended by 10 ft (3.0 m); a much larger rear fuselage, vertical tailfin, rudder, and horizontal stabilizer were added; a gunner's position was added in the new tail; the nose (especially the bombardier's well-framed, 10-panel nose glazing) remained relatively the same as the earlier B through D versions had; a Sperry electrically powered manned dorsal gun turret just behind the cockpit was added; a similarly powered (also built by Sperry) manned ventral ball turret just aft of the bomb bay – replaced the relatively hard-to-use, Sperry model 645705-D remotely operated ventral turret on the earliest examples of the E variant.  These modifications resulted in a 20% increase in aircraft weight. The B-17's turbocharged Wright R-1820 Cyclone 9 engines were upgraded to increasingly more powerful versions of the same powerplants throughout its production, and similarly, the number of machine gun emplacement locations was increased.
  The B-17F variants were the primary versions flying for the Eighth Air Force to face the Germans in 1943, and had standardized the manned Sperry ball turret for ventral defense, replacing the earlier, 10-panel well-framed bombardier's nose glazing from the B subtype with an enlarged, nearly frameless plexiglas bombardier's nose enclosure for improved forward vision.
  Two experimental versions of the B-17 were flown under different designations, the XB-38 Flying Fortress and the YB-40 Flying Fortress. The XB-38 was an engine test bed for Allison V-1710 liquid-cooled engines, should the Wright engines normally used on the B-17 become unavailable. The only prototype XB-38 to fly crashed on its ninth flight, and the type was abandoned. The Allison V-1710 was allocated to fighter aircraft.
  The YB-40 was a heavily armed modification of the standard B-17 used before the North American P-51 Mustang, an effective long-range fighter, became available to act as escort. Additional armament included an additional dorsal turret in the radio room, a remotely operated and fired Bendix-built "chin turret" directly below the bombardier's accommodation, and twin .50 in (12.7 mm) guns in each of the waist positions. The ammunition load was over 11,000 rounds. All of these modifications made the YB-40 well over 10,000 lb (4,500 kg) heavier than a fully loaded B-17F. The YB-40s with their numerous heavy modifications had trouble keeping up with the lighter bombers once they had dropped their bombs, so the project was abandoned and finally phased out in July 1943. The final production blocks of the B-17F from Douglas' plants did, however, adopt the YB-40's "chin turret", giving them a much-improved forward defense capability.
  By the time the definitive B-17G appeared, the
caption: Postwar SB-17G-95DL (ser. no. 44-83722), assigned to the 2nd ERS as a search-and-rescue aircraft, beside a Stinson L-5
exif: {}