Klimov M-100, was reserved inside the fuselage, in a fairing above the wing spars and behind the pilots. It was intended to drive a supercharger that supplied pressurized air to the Mikulin AM-34FRN engines, with the installation designated ATsN-2 (—Central Supercharging Unit), an idea pioneered in 1918 by the Zeppelin-Staaken firm in the German Empire, and refined further for the Third Reich Luftwaffe's Do 217P and Hs 130E experimental bomber designs. Subsequent models of the Pe-8 omitted the internal engine, and provided seating for a flight engineer and radio operator, behind and below the pilots. The bombardier sat in the nose and manned a turret armed with a ShVAK cannon that covered a 120° cone ahead. A prominent chin gondola, nicknamed the 'beard', protruded beneath the nose. The dorsal gunner sat at the rear of the ATsN fairing with a sliding hood covering a ShKAS machine gun and another ShKAS mounted in a ventral hatch. The tail gunner had a powered turret with a ShVAK and, most unusually, there were manually operated ShVAK cannon mounted at the rear of each inner engine nacelle. Crewmen had access to these positions through the wing or by a trapdoor in the upper wing surface. The large internal bomb bay racks held up to of bombs; external racks held a single FAB-500 (Fugasnaya AviaBomba - high explosive bomb) bomb under each wing. The maiden flight of the unarmed prototype, piloted by M. M. Gromov and without the ATsN installation, occurred at Khodynka Aerodrome on 27 December 1936. After successful initial trials, the ATsN system was installed for the State acceptance trials in August 1937 and the AM-34RNB engines were fitted during the tests. Gromov reported that the rudder was ineffective and that the outer engines overheated. Subsequent wind tunnel testing identified a problem with the aerodynamics of the radiators and nacelles. To solve this problem, the outer engines' radiators were moved into deep ducts under the inner nacelles. The Pe-8 now featured only two pronounced radiator intakes, one under each inner engine, each shared by both inner and outer engines, one of the distinctive and unique features of the aircraft. The rudder was also enlarged and redesigned with a smooth skin. Construction of a second prototype began in April 1936, incorporating lessons from the first aircraft and feedback from the VVS. Designers widened the fuselage by ; the 'beard' was also widened and the