B-17F

The B-17F ushered in a new era for the "Flying Fortress" design, becoming the first variant to be produced in truly massive numbers. Boeing alone manufactured 2,300 aircraft at their Seattle facility, while Douglas and Lockheed Vega contributed an additional 1,105 units - 605 and 500 respectively.

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This dramatic ramp-up in B-17F production was enabled by several key improvements over earlier models. Most notably, the aircraft featured a redesigned nose section and strengthened landing gear to accommodate a higher gross weight. Other enhancements included increased fuel capacity, additional armor protection, the provision of external bomb racks beneath the inner wings, and - on late-production examples - the adoption of more powerful R-1820-97 radial engines.

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These comprehensive upgrades transformed the B-17F into a substantially more capable and durable strategic bomber. Critically, the steps taken to streamline its manufacture allowed the aircraft's production to surge, ensuring the "Flying Fortress" could be fielded in the vast quantities required to overwhelm Axis air defenses.

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Introduction of Production Blocks

As the B-17F rolled off production lines in ever-greater numbers, the aircraft underwent a myriad of modifications across its various manufacturing blocks. To track these changes, the block numbers were set in increments of five - for example, Boeing produced a B-17F-1, followed by a B-17F-5, B-17F-10, and so on.

However, the specific block numbers did not align between the different manufacturers. Boeing's blocks reached as high as -130, while Vega topped out at -50, and Douglas went up to -80.

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To aid identification, each aircraft was assigned a manufacturer suffix - BO for Boeing, DL for Douglas, and VE for Vega. The way this worked in practice is that the suffix would be added to the alpha-numeric designation described above for a certain 'block' of serial numbers, such as B-17F-1-BO which covered serials 41-24340 to 41-24389; B-17F-5-BO covered serials 41-24390 to 41-24439 and so on.

 

This systematic approach to denoting the numerous engineering updates and the diverse production sources allowed the Army Air Forces to effectively manage the growing fleet of B-17Fs.

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This regimented modification and serialization process was essential as the "Flying Fortress" shifted into large-scale, high-tempo manufacturing. It ensured the Air Forces could efficiently track the evolving configuration of each B-17F, even as production was distributed across multiple industrial partners. This attention to configuration management would prove vital as the B-17F's pivotal role in the strategic air campaign continued to expand.

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Key Changes
One of the most distinctive features of the B-17F was its innovative one-piece molded Plexiglass nose cone, which eliminated the need for any internal metal support structure. This gave a more handsome aspect to the nose section of the B-17F and some think it made it the most attractive version.  The nose cone would be changed again on the B-17G, which introduced a shorter plexiglass dome to accommodate the chin turret elevation and muzzle blast.

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In early examples of the B-17F the original 0.30-inch nose gun assigned to the bombardier was retained, with four ball mounts for the gun to be moved between.

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B-17F 'The Flying Flit Gun' having its nose art applied.  The ball mounts for the 0.30inch gun in the new frameless plexiglass nose cone can be clearly seen

 

This 0.30 inch gun was later replaced with a larger 0.50-inch weapon, which was almost universally retrofitted in later B-17Fs until the chin turret was introduced on later produced examples.

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Further changes in the nose section were staggered square plexiglas positions for 0.50-inch machine guns on either side, This was introduced on some later produced B-17Es and was an instance of features being carried across onto later models.  At first the windows were square and flush with the fuselage, but later changed to faired examples as in the pictures below.

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Here we see the starboard cheek position on B-17F 'Sweetheart'.  The gun position is in front of the two smaller observation windows.  Also visible is the by now ubiquitous nose plexiglass mounted 0.50 inch machine gun

 

 

 

 

 

 

 

 

 

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Here we see the port cheek gun position on 'Old Ironsides'.  The position is located further back, in between the two observation windows.  The nose plexiglass mounted 0.50 inch machine gun is also more clearly visible, especially the interior bracing it was mounted on

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Here we see a later block B-17F 'Windy City Avenger' - the cheek gun is in the same position, but in a modified aerodynamic fairing that allowed a better forward field of fire

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The portside cheek gun on a later block B-17F, clearly showing the new faired mount.  The light reflecting off the plexiglass shows the extent of the bulge required to meet the front fairing.  This gave a better field of view than the earlier flush mounts

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Indeed, as the air war in Europe progressed, the nose armament in the B-17F had become a veritable patchwork of configurations - from twin 0.50-inch guns to fixed forward-firing 0.50-inch weapons that could be engaged by the pilot. These extensive updates were driven by the Luftwaffe's determined attacks on the B-17's nose, long identified as its weakest defensive point. A final solution only came about with the introduction of the chin turret; first used in the experimental YB-40 programme, installed on later B-17Fs, before being standardised on the B-17G.

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B-17F 'Yankee Gal' with twin fifties mounted in the nose plexiglass.  Th extra bracing may be seen below the guns

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Of equal significance was the B-17F's incorporation of additional "Tokyo tanks" in the outer wing panels. These auxiliary fuel cells carried an extra 915.9 Imperial gallons (4,165 liters) of fuel, dramatically extending the bomber's operational range.

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Weight Increases

The B-17F's empty weight grew substantially, from 27,500 pounds for the D model and 32,250 pounds for the E, up to 34,000 pounds. With a maximum authorized gross weight of 56,500 pounds, this left ample capacity for a full bomb load - often exceeding 10 tons in practice, despite the given limit.

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However, on typical combat sorties, the actual bomb load more realistically ranged around 5,000 pounds (2,268 kg). This balanced approach ensured the B-17F could carry a meaningful offensive payload while maintaining the fuel reserves and structural margins needed for the bomber's high-altitude, long-endurance missions.

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Matching Performance Increases

From an operational standpoint, the chief advantage of the B-17F over the previous E model was its improved Wright Cyclone engines, which drove larger-diameter propellers with wider blades. These powerplants provided greater maximum horsepower at the optimum combat altitude of 25,000 feet, where a test aircraft achieved an impressive top speed of 325 mph - the fastest of all production Fortresses.

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Therefore, although weighing nearly 50% more than the original Model 299 from 1935, the B-17F nonetheless delivered remarkably similar performance to other contemporary heavy bombers. However, its major advantage lay in the aircraft's superior combat ceiling, which allowed it to operate with ease at 25,000 feet - far higher than the RAF's Stirling, which struggled to maintain 15,000 feet.

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In addition to this enhanced performance, the B-17F incorporated numerous other detail improvements. These included important armament upgrades, such as the adoption of ammunition trace feed systems in place of bulky containers. The aircraft also benefited from additional armor protection for the crew and engines - enhancements that inevitably increased the overall empty weight.

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These complementary upgrades to the B-17F's power, armament, and protection transformed it into the definitive heavy bomber variant. The combination of heightened performance, lethality, and resilience made the B-17F an increasingly formidable instrument of air superiority in the relentless strategic bombing campaign over Europe.

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Yet, this lofty operating altitude no longer provided the "Flying Fortress" the same degree of immunity from enemy fighters and anti-aircraft fire as originally envisioned during its development. To survive the increasingly deadly daylight bombing missions over Europe, the B-17 would have to live up to its reputation as a true "Flying Fortress" - relying on its formidable defensive armament and structural resilience.

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