The Boeing B-29 Superfortress, the most technologically advanced airplane produced during World War II, first flew Sept. 21, 1942. The B-29 had many new features, including guns that could be fired by remote control. The crew areas were pressurized and connected by a long tube over the bomb bays. The tail gunner had a separate pressurized area that could only be left during unpressurized flight. At 105,000 pounds, the B-29 was also the heaviest production plane because of increases in range, bomb load and defensive requirements. The B-29 used the high-speed Boeing 117 airfoil, and its larger Fowler flaps added to the wing area as they increased lift. Modifications led to the B-29D, upgraded to the B-50, and the RB-29 photo reconnaissance aircraft. The Soviet-built copy of the B-29 was called the Tupolev Tu-4. A total of 3,970 B-29s were built.