The B-17F was an upgrade of the B-17E, although outwardly the types were distinguished only by exchanging the framed nose-glass for a molded one-piece plexiglas cone. Late production "F" series aircraft received a "cheek"-mounted gun on each side of the nose, and fully-feathering paddle-bladed propellers. Numerous internal changes were made to improve the effectiveness, range, and load capacity of the B-17. However, once placed in combat service, the "F" series was found to be tail heavy. The weight of gunners and ammunition when combat-loaded moved the center of gravity rearward from its design point and forced the constant use of elevator trim tab, stressing this component. In combat the B-17F proved almost immediately to have inadequate defensive protection when attacked from the front. Various armament configurations of two to four flexible guns were utilized in the field, but the problem was not adequately addressed until introduction of a powered "chin" turret.
By using a stronger undercarriage, the maximum bomb capacity was increased from 4,200 lb ( kg) to 8,000 lb ( kg). Though this modification reduced cruise speed by 70 mph (100 km/h), the increase in bomb capacity was a decided advantage. A number of other modifications were made, including re-integrating external bomb racks, but because of its negative impact on both rate-of-climb and high altitude flight the configuration was rarely used and the racks were removed.
Range and combat radius were extended with the installation in mid-production of additional fuel cells in the wings. Called "Tokyo tanks", nine self-sealing rubber-composition tanks were mounted inside each wing on each side of the joint between the inner and outer wing sections. With an extra 1,080 gal (4,088 l) to the 1,700 gal (6,435 l) available on the first B-17Fs, the Tokyo tanks added approximately 900 mi (1,448 km) to the bomber's range.
3,405 were built: 2,300 by Boeing, 605 by Douglas, and 500 by Lockheed (Vega). These included the famous Memphis Belle. 19 were transferred to the RAF, where they served with RAF Coastal Command as the Fortress II.