BIRTH OF THE B-17
In the early summer of 1934, the US Army Air Corps released a requirement for a multi-engined medium bomber intended primarily for the coast-defence role. The requirement called for the ability to deliver a 2000 lb (907 kg) bomb load over a range of at least 1020 miles (1641 km) but preferably 2200 miles (3540 km) at a speed of at least 200 mph (322 km/h) but preferably 250 mph (402 km/h). Boeing had already developed its Models 214, 215 and 246 series of closely related monoplanes for limited use as the B-9 series of twin-engined experimental and service test bombers, and fully appreciated that the monoplane layout of these aircraft offered little scope for improvement in its twin-engined form given the relative lack of power available from contemporary radial piston engines, or those foreseeable in the immediate future. The design team therefore chose to construe the USAAC’s multi-engined specification as meaning not necessarily just two engines as adopted by other contenders.
The design team decided that a three-engined powerplant, with an engine in the nose, was impractical, and therefore agreed a four-engined design. The design team began work in June 1934 on the Model 299, and construction of the prototype began in August of the same year as the start of a programme that was to produce one of the most important warplanes of all time. At the time it was designing the Model 299, Boeing was also at work on the considerably larger Model 294 (XBLR-1, later XB-15) bomber prototype, and the Model 299 can be regarded as an aerodynamic and structural cross between the Model 247 transport and Model 294 bomber. From the former came the basic structural design and from the latter the disposition of the four-engined powerplant, the circular-section fuselage, and the arrangement of the crew, weapons (both defensive and offensive) and other military equipment within the fuselage. In terms of size the Model 299 was about midway between the Model 247 and Model 294, and its span was only 8 ft 3 in (2.51 m) greater than that of its most significant rival, the military derivative of the twin-engined Douglas DC-3 transport that was designed as the DB-1 and developed into the B-18 Bolo bomber. The Model 299 prototype was a company-owned aeroplane, and first flew in July 1935 with a powerplant of four Pratt & Whitney R-1690-S1EG Hornet radial engines each rated at 750 hp (559 kW), a crew of eight (two pilots, bombardier, navigator/radio operator and four gunners), an offensive load of 4800 lb (2177 kg) with eight 600 lb (272 kg) bombs carried internally, and a defensive armament of five 0.3 in (7.62 mm) Browning trainable machine guns in a small nose turret and in four blister fairings (one dorsal, one ventral and two beam positions).
At this time, the USAAC had little funding and, thus lacking the ability to order the Model 299 into large-scale production, proceeded by ordering small batches, each introducing a significant improvement in capability over its predecessor. The 13 Y1B-17 service test aircraft were therefore followed by one example of the Y1B-17A, 39 examples of the B-17B, 38 examples of the B-17C and 42 examples of the B-17D, before the emergence of the first large-scale production variant, the B-17E, that resulted directly from the lessons of air operations over Europe in the opening campaigns of the war. The revised type was known to the manufacturer as the Model 2990, and its most important changes from the Model 299H (B-17C and B-17D) were a completely revised rear fuselage carrying larger tail surfaces, improved armour protection, a number of internal enhancements, and revised defensive armament. This last was particularly important: the waist positions were simplified, power-operated Sperry turrets each armed with two 0.5 in (12.7 mm) machine guns were added in the dorsal and ventral positions, and a manually operated tail turret had a further pair of 0.5 in (12.7 mm) machine guns. The first B-17E flew in September 1941, and the initial 112 aircraft off the production line featured a ventral turret that was remotely controlled from a periscopic sight in a Plexiglas blister located several feet farther to the rear; from the 113th aeroplane onwards, this installation was replaced by a Sperry ball turret accommodating a gunner. Other major changes included a powerplant of four R-1820-65 radial piston engines each rated at 1200 hp (895 kW).
FIRST B-17E DELIVERIES
The first B-17E bombers were delivered to the 7th Bombardment Group that joined the 19th Bombardment Group in the Pacific theatre from December 1941, and others of the 512 aircraft were allocated to the units that formed the first elements of the 8th Army Air Force in the UK from May 1942. The first British-based unit to become operational with the B-17E was the 97th Bombardment Group, which flew its first mission over Europe in August 1942. Other medium-range raids, mainly to targets in occupied North-West Europe, followed into the early part of 1943, but much of the European-based B-17E strength was diverted, from October 1942, to create the striking element of the new 12th Army Air Force that was to support the Anglo-American landing in Northwest Africa during November 1942 and the subsequent campaign up to the final elimination of the Axis forces from Africa by May 1943. Some 45 B-17E bombers were transferred to the RAF in late 1942 for use by Coastal Command designated as Fortress Mk IIA (the designation Fortress Mk I had been used for 20 B-17C aircraft) and, with the newer B-17F that had been delivered to the RAF earlier than the B-17E thus receiving the designation Fortress Mk II, served with four maritime and four meteorological reconnaissance squadrons.
The B-17F resulted from direct American operational experience, in this instance with the B-17D against the Japanese. The B-17F was externally distinguishable from the B-17E only by its single-piece blown rather than multi¬ piece built-up Plexiglas nose transparency, but the variant in fact incorporated more than 400 subtler but collectively very important changes that made the B-17F an altogether more formidable warplane. It was powered by four R-1820-97 radial piston engines each rated at 1200 hp (895 kW). Changes were added incrementally throughout the B-17F’s production life, including improved armour protection, provision for external bomb racks under the wing, increasing the maximum possible bomb load to 20,800 lb (9435 kg) for short-range missions, additional ball-and-socket machine gun mounts in the nose and the radio compartment for an extra three 0.5 in (12.7 mm) trainable machine guns, an electronic link between the Norden bomb sight and the autopilot, changed control settings, more photographic equipment, a revised oxygen system, upgraded main landing gear units allowing an increase in maximum take-off weight to 65,000 lb (29,484 kg) and eventually to 72,000 lb (32,659 kg), a dual braking system, self-sealing oil tanks, extra electrical power generation capability, dust filters over the carburettor air inlets, provision for 909.3 Imp gal (4133.7 litres) of auxiliary fuel in `Tokyo tanks’ installed in the wings, and paddle-blade propellers for the R-1820-97 radial piston engines but installed in revised nacelles to allow the full feathering of the wider-chord props.
The initial B-17F flew in May 1942, just two days after the delivery of the last B-17E, and was the first Flying Fortress to be built in the block system adopted by the USAAF in 1942 to differentiate minor improvements of standard introduced on the production line, but not meriting a change in the basic letter suffix. Production totalled 3405 aircraft in the form of 2300 from Boeing in 28 blocks, 605 from Douglas in 18 blocks, and 500 from Lockheed’s Vega subsidiary in 11 blocks. The B-17F was allocated initially to the 8th Army Air Force in Europe, and flew the first American bombing mission against a target in Germany during January 1943. Thereafter the B-17F became the mainstay of the steadily increasing US daylight bombing effort, but operations from mid-1943 revealed that the Germans were becoming wise to American tactics and also to the defensive limitations of the B-17F. The crisis in this variant’s career arrived in August and October 1943 with two major bombing efforts that resulted in very heavy American losses. The first was a raid by 376 aircraft on factories at Schweinfurt, Wiener Neustadt and Regensburg, and the second a raid by 291 aircraft on Schweinfurt. In each case the attacks lost some 60 aircraft as the Germans waited until the US escort fighters had turned back and then attacked the poorly coordinated bomber streams with a succession of fighters that attacked head-on against the bombers’ most vulnerable defensive arc.
The final B-17G resulted directly from the experience of the US bomber crews in 1943, which revealed that the B-17F lacked adequate defence against head-on fighter attack. The primary change in the B-17G was therefore the introduction of a power-operated Bendix chin turret armed with two 0.5 in (12.7 mm) machine guns and controlled remotely from the glazed nose position that was now a more practical unit as it lost the one or two manually operated 0.5 in (12.7 mm) machine guns that had been fitted in the B-17F. These weapons had not enjoyed adequate fields of fire to be truly effective, and had also made movement in the nose position very difficult. Other changes effected successively in the B-17G were an improved navigator position, refinement of the bomb-control system, electric rather than hydraulic turbocharger control, improved turbochargers, an emergency oil supply for propeller feathering, improved cockpit instrumentation, a primitive form of electrical power boosting for the control column as a means of reducing pilot workload during heavily-laden formation flight, and further upgrading of the defensive armament. This last was effected by alterations to the waist and tail positions: the former were fitted with fixed windows that deflected the icy airflow that otherwise affected the two gunners, and then staggered longitudinally so that each of the two gunners could move freely without impeding the other; the latter became known as the `Cheyenne turret’ when modifications that reduced overall length by 5 in (0.127 m) increased the tail turret’s field of fire by an appreciable degree and replaced the original ring-and-bead sight with a reflector sight. The single gun was later removed from the navigator’s position, whose hatch mounting was deemed to provide inadequate fields of vision and fire.
The first B-17G was delivered in September 1943, and production lasted to April 1945, when the 8680th B-17G was delivered to end Flying Fortress production after the completion of 12,731 aircraft. The B-17G was the most prolific of all Flying Fortress variants: each of the three manufacturers produced the type in 23 blocks, deliveries from Boeing, Douglas and Vega amounting to 4035, 2395 and 2250 aircraft respectively.
Very much like the Halifax and Lancaster its blind spots were picked out early. For the 17F this was dead on from the front and a little from below leaving the upper turret useless being unable to depress far enough, the ball turret was too far back to shoot properly to the front and the two guns in the nose didn’t point ahead. Made sense to add the chin turret.
The American Bombers in all theaters and all heavy bombers quickly found out that the enemy’s head on attacks were really hard to defend against with single flexible .50 brownings. The head on attacks were a great tactic for the fighters because when you are going 300mph and the bomber is going 250mph, you are closing on each other head on at 500 plus mph and it limited the ability of the bomber gunners to return fire. That being said, the chin turret was added to counter this major threat and add highly focused firepower to the front to make the fast closing head on attacks less fun for the fighters. The same thing happened with the B-24 bombers in the pacific, so the mechanics in the field started taking the rear B-24 turrets and adding them to the front of the B-24 aircraft and it worked great for them too. I can’t imagine how much work it took to add a turret in the field like that with the extreme modification to the front of the aircraft, including a larger area added underneath the turret. With the B-24 front turret, they couldn’t add them too the production line quick enough, so the new aircraft would get sent to modification centers across the country to get the front turret added and the guys no longer had to jerry rig turrets into the aircraft in the field.
The idea for the chin turret of the B-17G actually came from the YB-40 gunship concept during the time period of the B-17F production run. The YB-40 gunship concept was a failure, but the B-17G inherited not only the chin turret, but the offset waist gunner positions and the improved tail gun station (referred to as a “Cheyenne”) from the YB-40.
As a side note, the 13 .50 caliber guns of the B-17G paled in comparison to the YB-40’s average of 16 .50 guns, but a few were known to have been modified to carry more than this number.