tail section was modified to lessen resistance and improve rudder function. A reconfigured control system included an autopilot and the engineers redesigned portions of the electrical system. The engines were changed to the more powerful AM-34FRNVs and a redesigned undercarriage was fitted to the airframe. Two additional fuel tanks increased the craft's range. The defensive and offensive armament was revised, and the bomber's weaponry expanded to twin ShKAS guns in the nose, nacelle barbettes and tail turrets and a dorsal turret with a ShVAK; this design eliminated the ventral gun. The bomb bay was modified to allow for a single FAB-5000 bomb to be carried and provisions were added to carry VAP-500 or VAP-1000 poison gas dispensers under the wings. The arrests of both Tupolev and Petlyakov in October 1937, during the Great Purge, disrupted the program and the second prototype did not make its first flight until 26 July 1938. Although this prototype served as the basis for the series aircraft, further modifications were made to the armament. New weaponry included a retractable ShVAK in the MV-6 dorsal turret, another ShVAK in a KEB tail turret and a Berezin UBT machine gun in each ShU barbette in each inner engine nacelle, on the underside of the wing covering he lower rear arc of fire to left and right, respectively. Another fuel tank further increased the range, and the 'beard' was removed entirely, replaced by a more streamlined nose. Authorization for production was slow for several reasons, including the Great Purge, but also due to the scarcity of resources, and a shortage of workers. Although production facilities in the Kazan Factory No. 124 were ready as early as 1937, the order to begin was not given until 1939. Manufacture and supply problems Engine supply problems complicated the construction of the aircraft. Production of the ATsN superchargers could not be organized in any systematic way and only the first four Pe-8s were equipped with them. Factory No. 124 shut down its Pe-8 production line at the beginning of 1940 while alternative engines were evaluated. Somewhere in the massive Soviet chain of command, the decision was made to proceed without the superchargers. The unavailability of the Klimov M-100 engine of the ATsN-2 installation required a design change, although this modification allowed a commander and radio operator to be carried in its place. Then, to compound the problem further, the production of AM-34FRNV