Boeing B-17 Flying Fortress - Design and Variants

Design and Variants

It 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 turbo-supercharger 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 turbocharger, was re-designated 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 rudder and flaps. The B-17C changed from three bulged, oval shaped machine gun blisters to two flush, oval-shaped machine gun window openings and a single "bathtub" machine gun housing on the lower fuselage. Models A through D of the B-17 were designed defensively, while 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 tail fin, rudder, and horizontal stabilizer were added to the design; a gunner's position was added in the new tail; the nose (especially the bombardier's well-framed nose glazing) remained relatively the same as the earlier -B through -D versions had, but with the addition of an electrically-powered manned dorsal gun turret just behind the cockpit, and the similarly-powered Sperry manned ventral ball turret just aft of the bomb bay - replacing a relatively hard-to-use remotely operated ventral turret on the earliest examples of the -E variant, that had also been used on the earlier marks of the North American B-25 Mitchell - 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 multiple times throughout its production, and similarly, the number of machine gun emplacement locations were increased to enhance their aircraft's combat effectiveness.

The B-17F variants were the primary versions flying for the Eighth Air Force to face the Nazis in 1943, and had standardized the manned Sperry ball turret for ventral defense, along with an enlarged, nearly frameless Plexiglas bombardier's nose enclosure for much improved forward vision.

Two 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 testbed 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 V-1710 being kept for fighters.

The YB-40 was a heavily armed modification of the standard B-17 used before the 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 "chin turret" and twin .50 in (13 mm) guns in the waist positions. The ammunition load was over 11,000 rounds, making 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, and so the project was abandoned and finally phased out in July 1943, but not before the final production blocks of the B-17F adopting the YB-40's remotely operated and fired "chin turret" for a much-improved forward defensive weapons installation.

By the time the definitive B-17G appeared, the number of guns had been increased from seven to 13, the designs of the gun stations were finalized, and other adjustments were completed. The B-17G was the final version of the Flying Fortress, incorporating all changes made to its predecessor, the B-17F, adopting the remotely-operated "chin turret" for forward defense from the YB-40 "gunship" version, and in total 8,680 were built, the last one (by Lockheed) on 28 July 1945. Many B-17Gs were converted for other missions such as cargo hauling, engine testing and reconnaissance. Initially designated SB-17G, a number of B-17Gs were also converted for search-and-rescue duties, later to be redesignated B-17H.

Late in World War II, at least 25 B-17s were fitted with radio controls and television cameras, loaded with 20,000 lb (9,100 kg) of high-explosives and dubbed BQ-7 "Aphrodite missiles" for Operation Aphrodite. The operation, which involved remotely flying Aphrodite drones onto their targets by accompanying CQ-17 "mothership" control aircraft, was approved on 26 June 1944, and assigned the 388th Bombardment Group stationed at RAF Fersfield, a satellite of RAF Knettishall.

The first four drones were sent to Mimoyecques, the Siracourt V-1 bunker, Watten and Wizernes on 4 August, causing little damage. The project came to a sudden end with the unexplained mid-air explosion over the Blyth estuary of a B-24, part of the United States Navy's contribution as "Project Anvil", en route for Heligoland piloted by Lieutenant Joseph P. Kennedy Jr., future U.S. president John F. Kennedy's elder brother. Blast damage was caused over a radius of 5 miles (8.0 km). British authorities were anxious that no similar accidents should again occur, and the Aphrodite project was scrapped in early 1945.