It wasn't long before the shortcomings of the B-17E were addressed. With the introduction of the B-17F, over four-hundred changes had been made over the B-17E. Most of these changes were internal and very minor. However, all of them improved on the Flying Fortress's combat worthiness.
This is not to say there were not major changes to the aircraft. Externally one of the most noticeable changes was to the front nose cone. The metal framing was removed and was replaced with a single, molded Plexiglas nose that was longer than the original. The lower section of the nose cone retained the flat panel for bomb aiming. The longer nosecone gave the bombardier a better field of fire with his nose mounted machine gun and freed up some room in the nose compartment.
Another major innovation was the fitting of Paddle bladed propellers to the new Cyclone R-1820-97 engines. The propellers were 1 inch longer for an overall diameter of 11'7 inches. The blades were also widened through their cord letting them take a larger 'bite' out of the airstream and giving them their 'Paddle Bladed' look. Because of the wider propeller blade, the engine cowlings hand to be shortened and reshaped. This was necessary to allow the new propeller blade to fully feather unobstructed.
As was mentioned these were fitted to more power versions of the Write Cyclone engine. The R-1820-97 engines were rated at 1,380 horsepower under emergency wartime setting.
Part way through the production additional tanks were fitted to the outer wing panels of the B-17F. These tanks, most commonly referred to as Tokyo tanks, increased the B-17's fuel payload from 1,730 US gallons to 2,810 US gallons giving the B-17F tremendous range. These tanks appeared on B-17Fs starting with the B-17F-80-BO, B-17F-25-DL and B-17F-30-VE blocks.
It is important to address these block numbers and what they meant. With Boeing, Vega, and Douglas all building B-17s in different locations (Boeing was in Seattle, WA while Douglas and Vega were both in Southern California) it was impossible not to have differences in aircraft built on different production lines at different times and in different locations. It was thus decided to come up with a system to determine what model of B-17 the aircraft was along with when it was built and where it was built. Block numbers would begin with the model of the aircraft such as B-17F. Next would be the Block number itself and would be started with the number one. For example: B-17F-1 would be the first block of B-17Fs. These numbers would be increased by five per block so that the second block of B-17Fs would be numbered B-17F-5. The reason for this is so that if changes were made at the field modification centers that the planes were shipped too following production that these modifications could be tracked as well using the block number. Lastly, the plant ID was incorporated into the block number as well. BO was Boeing in Seattle, DL was the Douglas plant in Long Beach, CA, and VE was the Vega plant in Burbank, CA. With the B-17F, there were 27 line blocks from Boeing, 17 from Douglas, and 11 from Vega. So, B-17F-80-BO was the 12th block from Boeing, B-17F-25-DL was the 6th block from Douglas and B-17F-30-DL was the 7th block from Vega.
Other improvements to the B-17F were made to the landing gear, brakes, oxygen system, bomb racks, ball turret, bombsight/autopilot link, astro-compos, and a nose bubble in the upper nose section.
The armament changed in the B-17F as well. Side nose guns were added at the plants. These cheek guns had previously been fitted at modification centers. These were added in the B-17F-55-BO, B-17F-15-DL and B-17F-25-VE blocks. These guns also became staggered. The right side gun was moved forward while the left side gun was moved back. This allowed the Navigator (who usually would use the left gun) and the bombardier (who would operate the right) to keep from getting into each other's way. Later, these positions were bubbled out to allow the guns a greater forward field of fire against head on attacks. This bubbling out of the windows was carried out at modification centers on the F model.
Unfortunately, nose armament remained unchanged from the factory. The B-17F was still fitted with the small .30 caliber machine gun. However, combat in the Pacific and later in Europe showed that the enemy knew the front firepower of a B-17 was lacking and her crews decided to do something about it. Maintenance crews, at the request of the combat crews, fitted one, two, and sometimes three .50 caliber machine guns into the nosecone of the B-17F. The nose couldn't handle the recoil of a single .50 caliber machine gun much less three of them so additional bracing had to be provided to handle the additional recoil. Soon, some of these modifications started being done at modification centers in the states before the planes went overseas. While these temporary modifications were being made, the factories were trying to fine a more effective solution to the B-17's lack of forward firepower.
This solution was the Bendex Chin Turret. This turret had originally been used on the XB-40 gunship project. While this experiment proved unsuccessful, the chin turret was found to be a major improvement to the B-17's forward firepower. This turret was fitted to the last eighty-six B-17Fs to come off the Douglas assembly line starting with block B-17F-75-DL. Many times these last B-17Fs are mistaken for B-17Gs which were all equipped with this turret. The chin turret was fired by the Bombardier using a sight that hung from the top of the nose and protruded out into the nosecone. It was controlled by a yoke that could fold away to the right of the nose compartment in order to make room for the bombardier to use the Norden bombsight or was otherwise not in use. The turret housed two .50 caliber machine guns and was a welcomed addition. When Douglas and Boeing began to produce B-17s with this feature, the aircraft was re-designated as the B-17G.
The B-17F had some major performance differences over it's predecessors. Top speed was 325 mph. This was faster than any other production B-17 and was due to the new paddle bladed propellers and more powerful engines. Range, when equipped with Tokyo Tanks, was increased to 4,220 miles. Its surface sealing was raised to 38,000 feet again thanks to the new propellers. Max bomb load, using new external bomb racks on some B-17Fs, was 9,600 pounds though the average bomb load in the European theater was around 4,000 pounds.
In all, 2,300 B-17Fs were produced by Boeing while Douglas built 605 and Vega built 500. Douglas built Fs had stronger wing spars than their Boeing counterparts. Vega had similar wing structures and also featured reinforced fuselages. Because of this, Douglas and Vega built B-17Fs were slightly heavier than planes made by Boeing and thus had slightly different operational altitudes. Vega built B-17Fs had fuselage problems. They suffered stress constrictions where the fuselage was joined at the radio room. Wings from Douglas and Vega B-17Fs could not fit Boeing B-17Fs because of these production changes. Also, wings built in the higher temperature climate of Southern California were different from wings built in the cooler temperatures of Washington state. Because of their inexperience and thus lower efficiency at building the B-17, Douglas and Vega built aircraft were generally more expensive to produce than the Boeing versions.