use by the navigator. The problem of head-on defense was not adequately addressed until the introduction of a powered, Bendix-designed, remotely operated "chin" turret in the final production blocks of F-series Fortresses, starting with the last 65 (86 according to some sources) B-17Fs built by Douglas, from the B-17F-70-DL production block — directly derived from its debut on the YB-40 experimental "gunship" version. By using reinforced landing gear, the maximum bomb capacity was also increased from to . Though this modification reduced cruising speed by , increased bomb-carrying capacity was favored by decisionmakers over speed. A number of other modifications were made, including re-integrating external bomb racks; because of the negative impact on both rate-of-climb and high-altitude flight performance, this configuration was rarely used and the bomb racks were once again 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 fuel tanks were mounted inside each wing on each side of the reinforcing joint between the inner and outer wing spars. With an extra to the available on the first B-17Fs, the "Tokyo tanks" added approximately to the bomber's target capability. 3,405 "F" series Flying Fortresses 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. Three examples of the B-17F remain in existence, including the restored Memphis Belle. B-17G All changes made to the Flying Fortress were incorporated into the final production version, the B-17G. These included the Bendix remotely-operated chin turret, bringing the bomber's defensive armament to thirteen .50 caliber machine guns. The waist gun windows were staggered, another carryover from the YB-40 "gunship" variant. This allowed more freedom of movement for the waist gunners. The earliest B-17Gs lacked the "cheek" machine gun mounts, as it was believed that the chin turret provided sufficient forward firepower; they were quickly reintroduced when this proved untrue. In a reversal of the B-17F's design, the starboard "cheek" machine gun mount was moved rearward and the port side mount was moved forward, just behind the edge of the bombardier's nose glazing to avoid interference with the storage of the chin turret's control yoke when it was not in use. For late production blocks of the G-series, the tail gun position