series were built, and the 18 remaining B-17Cs were converted to Boeing's new B-17D standard. The sole-surviving example of the "D" series (originally built in 1940 and nicknamed Ole Betsy by her original aircrew) is currently undergoing restoration at the National Museum of the United States Air Force in Dayton, Ohio. This B-17D was later renamed "The Swoose" by her last pilot Col. Frank Kurtz, who after the war, kept the Fortress from being scrapped; he later named his daughter, actress Swoosie Kurtz, after the bomber. B-17E The B-17E (299-O) was an extensive redesign of the previous B-17D. The most obvious change was the larger, completely new vertical stabilizer, originally developed for the Boeing 307 Stratoliner by George S. Schairer. The new fin had a distinctive shape for the time, with the opposite end of the fuselage retaining the ten panel bombardier's nose glazing from the B-17D. Because experience had shown that the Flying Fortress would be vulnerable to attack from behind, both a tail gunner's position and a powered, fully traversable dorsal turret behind the cockpit, (each armed with a pair of "light-barrel" Browning AN/M2 .50 cal. machine guns), were added to the B-17E. Until this modification, aircrews had to devise elaborate maneuvers to deal with a direct attack from behind, including swinging the bomber laterally, allowing the waist gunners to alternate .50 caliber bursts at enemy fighters. (The configuration of a "3-window box" would later be implemented on the B-29, and also adopted by Soviet bombers as late as the Tupolev Tu-16 Badger, and in different form on the USAF's B-52). The teardrop-shaped sliding panels of the waist gunners were replaced by rectangular windows, located directly across the fuselage from each other, for better visibility. In the initial production run, the ventral "bathtub" machine gun emplacement of the B-17C/Ds was replaced by a remotely-sighted powered turret. It was similar to the one used as a ventral fuselage-mount Bendix remote turret of the B-25B through -D Mitchell medium bomber variants, but was difficult to use and proved to be a failure in combat. This resulted in all remaining B-17E production being fitted with a powered Sperry ball turret, manually operated from inside. These ball turrets also equipped the "F" and "G" series Flying Fortresses that followed for ventral-quarter defense. A total of 512 were built (possibly from the July 1940-dated order from the then-USAAC for B-17s being for that