List of Boeing B-17 Flying Fortress variants
The following is an extensive catalogue of the variants and specific unique elements of each variant and/or design stage of the Boeing B-17 Flying Fortress, a heavy bomber used by the United States Army Air Forces and other Allied air forces during World War II.
Boeing Model 299 (B-17)
The Model 299 was the original aircraft built by Boeing to fulfill an August 1934 requirement by the United States Army Air Corps for a bomber capable of carrying of bombs at. The 299 was powered by four Pratt & Whitney S1EG Hornet radial engines, giving a maximum speed of and a maximum gross weight of. It carried a bomb load of up to eight bombs, with a defensive armament of five machine guns, with one in a nose turret and one each in dorsal and ventral mounts and two in waist blisters. In 1935, Boeing's Model 299 competed with entries from other aircraft companies at an evaluation at Wright Field near Dayton, Ohio, USA.On its flight from Seattle, Washington to Wright Field for the competition, the 299 set a nonstop speed record of. Though it crashed and burned on takeoff during a demonstration, the crash was due to flight-crew error, when unreleased gust locks prevented flight surfaces from moving, not from any technical flaw in the aircraft. Subsequently, a mandatory pre-flight checklist prior to take-off was required. Despite the crash and its much higher unit cost, the Army Air Corps leaders was impressed by its performance, so Boeing was awarded a development contract. The aircraft has since been referred to as the XB-17, but this designation was not contemporary or official.
Y1B-17 (YB-17)
Though still enthusiastic about the Boeing design, despite it being disqualified from the fly-off contest following the crash of the Model 299 prototype, the Army Air Corps cut its order from 65 service test YB-17s to just 13. On November 20, 1936, the bomber's normal acquisition funding was changed to "F-1", indicating funding outside of normal annual allocations, and the heavy YB-17 bomber was redesignated "Y1B-17", as a result.Unlike the first example, which had used Pratt & Whitney R-1690 Hornet radial engines, the Y1B-17 used the more powerful Wright R-1820 Cyclone that would be the standard power plant for all subsequent B-17s. Changes were also made in the armament, and the crew was reduced from seven to six. Most changes were minor: the most notable was switching from double-wishbone to single-arm landing gear for ease of maintenance.
On 7 December 1936, five days after the first flight of the Y1B-17, the brakes fused during landing, and it nosed over. Though damage was minimal, the cumulative impact of this event, combined with the crash of the Model 299, triggered a Congressional investigation. Following the crash, the Army Air Corps was put on notice and another crash would mean the end of the bomber's "F-1" procurement program.
The commander of Army General Headquarters, Major General Frank Andrews, assigned twelve Y1B-17s to the 2nd Bomb Group located at Langley Field, Virginia to develop heavy bombing techniques. Of the thirteen Boeing aircraft built, one was assigned for stress testing. Most of the time spent with the bombers entailed eliminating problems with the aircraft but the most important development was the use of a detailed checklist reviewed by the pilot and copilot prior to each takeoff. It was hoped that this procedure would prevent further accidents.
In May 1938, the Y1B-17s of the 2nd Bombardment Group, led by the lead bombers' navigator Curtis LeMay, took part in a demonstration in which they intercepted the Italian liner Rex. Coming into contact with the liner while it was still out at sea, the demonstration was meant to prove the range and capabilities of the B-17. It also showed that the bomber could attack a naval invasion force before it could reach land. The Navy was furious about the Army's intrusion into their mission, and forced the War Department to issue an order restricting the Army Air Corps from operating more than a hundred miles from America's coastline.
After three years of flight, no serious incidents occurred with the B-17s. In October 1940, they were transferred to the 19th Bombardment Group at March Field.
B-17A (Y1B-17A)
The aircraft that became the sole Y1B-17A was originally ordered as a static test bed. However, when one of the Y1B-17s survived an inadvertent violent spin during a flight in a thunderhead, Army Air Corps leaders decided that there would be no need for static testing. Instead, it was used as a testbed for improving engine performance. After studying a variety of configurations, use of a ventral-nacelle-mount turbocharger was chosen for each of its four engines. A successive series of General Electric-manufactured turbochargers would equip B-17s as standard items, starting with the first production model, allowing it to fly higher and faster than the Y1B-17. When testing was completed, the Y1B-17A was reconfigured as a B-17A.B-17B
The B-17B was the first production model of the B-17 and was essentially a B-17A with a slightly larger rudder, larger flaps, a redesigned nose and R-1820-51 engines. The small, globe-like, machine gun turret used in the Y1B-17's upper nose blister was replaced with a machine gun, its barrel run through a ball-socket in the heavily framed Perspex nose glazing. The Y1B-17's separate triangular-shaped bombardier's aiming window, located further back in the lower nose in an indent, was eliminated, and was replaced with a framed window panel in the lower portion of the nose glazing. All B-17B aircraft were later modified at Boeing, being brought up to the B-17C/D production standard. While the new nose glazing still used only a single caliber machine gun, two additional ball-sockets were installed in the nose, one in the upper left panel and another in a lower right. This configuration was continued up through the B-17E series. During Army Air Corps service, the bulged teardrop-shaped machine gun blisters were replaced with flush-mounted Perspex side windows of the same type used in the B-17C/D series. Various aircraft had different levels of upgrades performed. Some of the "B" series Fortresses had only their bulged side blisters replaced with slide-out flush windows, while others also had their bulged upper blister changed to a much flatter, more aerodynamic Perspex window panel. In addition, some "B" series Fortresses also had ventral "bathtub turrets" installed, replacing their lower, teardrop-shaped gun blisters.Crew locations were rearranged, and the original pneumatic brakes were replaced with hydraulic brakes.
In October 1942 all in-service B-17B aircraft were redesignated RB-17B, the "R" indicating "Restricted" and these aircraft were now used only for training and transport duties. The "R" prefix designated combat obsolescence.
Many of these RB-17B aircraft, along with at least one still-airworthy YB-17, were stationed at Sebring Airfield, where the exterior scenes were filmed for the Warner Bros. war drama Air Force, directed by Howard Hawks, and starring John Garfield, Arthur Kennedy, Gig Young, and Harry Carey. The film's real star, however, was an RB-17B. It passed as a later model B-17D Flying Fortress, having had its machine gun blisters replaced and a lower "bathtub" ventral gun turret installed. Many of these aircraft can be seen in both ground and aerial scenes during the film.
The "B" series Flying Fortress first flew on 27 June 1939. Thirty nine were built in a single production run, but Army Air Corps serial numbers were scattered over several batches. This was because of limited government funding: The Army Air Corps could only afford to purchase a few B-17Bs at a time.
B-17C
The B-17C introduced a number of improvements over the B-17B, including more powerful Wright R-1820-65 engines. To improve crew safety, the waist-mounted machine gun blisters were replaced with teardrop-shaped, slide-out Perspex window panels flush with the fuselage, and the ventral blister was replaced by a lower metal gondola housing dubbed a "bathtub turret". The most important additions made to the "C" series were self-sealing fuel tanks and armor plate.The first of the B-17C series flew in July 1940, with 38 being built. The 18 remaining in Army Air Forces service, after 20 had been transferred to the RAF, were upgraded to the B-17D configuration, although one of these crashed on a ferry flight in the US.
Fortress Mk.I
With the passage of the Lend-lease Act in 1941, the Royal Air Force requested B-17s. At that time, the US Army Air Corps was suffering from shortages, but supplied 20 to the RAF. Though the Air Corps hadn't cleared the B-17 for combat, they were desperately needed in Britain. The 20 ferried bombers were production B-17Cs, designated Fortress Mk I by the RAF. The aircraft's single.30 caliber nose-mounted machine guns were replaced with.50 caliber Brownings.Following delivery, the 20 Fortress Mk.Is went almost immediately into frontline service, however they performed poorly. By September 1941, 39 sorties had only resulted in 22 missions, with nearly half of the sorties aborted due to mechanical and electrical problems. Eight of the 20 aircraft had been destroyed by September, half to accidents. Their guns froze-up at altitude and were unable to protect the Fortresses from attack and their effectiveness as bombers was also limited, largely because of problems with achieving an adequate level of bombing accuracy.
B-17D
Though changes in the design made the Army Air Force decide that the B-17D was worthy of a new sub designation, the B-17C and B-17D were very similar. In fact, both were given the same sub designation by Boeing.Minor changes were made, both internally and externally. Outside, the engines received a set of adjustable cowl flaps for improved cooling, and the externally-mounted bomb racks were removed. On the interior, the electrical system was revised, and another crew position was added, bringing the total number to ten. In the aft-dorsal radio compartment was a new overhead twin-.50s machine gun mount; in the central-aft section's ventral "bathtub" gun position, twin.50s were also added, as was additional armor plating. Nose gun ball sockets were added to the side windows for the first time, in a longitudinally staggered layout. The number of machine guns aboard brought the total armament to seven: one portable nose and six. The B-17D also featured more extensive armored plate protection. A total of 42 "D" series were built, and the 18 remaining B-17Cs were converted to Boeing's new B-17D standard. The sole-surviving example of the "D" series is currently undergoing restoration at the National Museum of the United States Air Force in Dayton, Ohio. This B-17D was later renamed "The Swoose" by her last pilot Col. Frank Kurtz, who after the war, kept the Fortress from being scrapped; he later named his daughter, actress Swoosie Kurtz, after the bomber.