Editing Petlyakov Pe-8 (section)

==Design and development==
Development of the Pe-8 began in July 1934, when the [[Soviet Air Forces]] (VVS) issued requirements for an aircraft to replace the obsolete and cumbersome [[Tupolev TB-3]] heavy bomber. These requirements specified a bomber that could carry {{convert|2000|kg|abbr=on}} of bombs {{convert|4500|km|abbr=on}} at a speed greater than {{convert|440|km/h|abbr=on}} at an altitude of {{convert|10000|m|0}}, figures that were twice the range, speed and service ceiling of the TB-3.<ref>''Pe-8: Last of a Generation'', ''Air International'', p. 80</ref> The task was assigned to the [[Tupolev]] Design Bureau ([[OKB]]) where [[Andrei Tupolev]] handed the work to a team led by Vladimir Petlyakov and the project received the internal bureau designation of ANT-42. The resulting aircraft, a four-engined, mid-wing [[cantilever]] [[monoplane]], was initially designated as the TB-7 ({{langx|ru|Тяжёлый Бомбардировщик}}, ''Tyazholy Bombardirovschik''—Heavy Bomber) by the VVS and owed more to the streamlined design of the [[Tupolev SB]] than to the block-like design of the TB-3.<ref>Gunston, ''Tupolev Aircraft'', p. 98</ref>

The bomber was built mainly of [[duralumin]], with two steel [[spar (aviation)|spars]] in the wings, although the [[aileron]]s were fabric-covered. The pear-shaped [[monocoque]] fuselage required the pilots to sit in tandem, offset to the left. In the prototype, space for a fifth engine, an auxiliary [[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 ({{langx|ru-Latn|Agregat tsentral'novo nadduva}}—Central Supercharging Unit), an idea pioneered in 1918 by the [[Zeppelin-Staaken R.VI#The special "R.30/16" test aircraft|Zeppelin-Staaken]] firm in the [[German Empire]], and refined further for the Third Reich Luftwaffe's [[Dornier Do 217#Do 217P|Do 217P]] and [[Henschel Hs 130#Centrally-provided supercharging experiments|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 {{convert|20|mm|adj=on|sp=us}} [[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  {{convert|7.62|mm|sigfig=2|adj=on|sp=us}} [[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 {{convert|4000|kg|abbr=on}} of bombs; external racks held a single {{convert|500|kg|adj=on|sp=us}} [[FAB-500]] (''Fugasnaya AviaBomba'' - high explosive bomb) bomb under each wing.<ref>Gunston, ''Tupolev Aircraft'', pp. 98–99</ref><ref>Gunston, ''Osprey Encyclopedia'', pp. 278–279</ref><ref>''Pe-8: Last of a Generation'', ''Air International'', pp. 80–81</ref>

The maiden flight of the unarmed prototype, piloted by [[Mikhail Gromov (aviator)|M. M. Gromov]] and without the ATsN installation, occurred at [[Khodynka Aerodrome]] on 27 December 1936.<ref>Gunston, ''Tupolev Aircraft'', pp. 99–100</ref> 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.<ref name=g75>Gordon (2005), p. 75</ref> 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.<ref name=g100>Gunston, ''Tupolev Aircraft'', p. 100</ref>

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 {{convert|100|mm|abbr=on}}; the 'beard' was also widened and the 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 {{convert|5000|kg|adj=on|sp=us}} [[FAB-5000 bomb]] to be carried and provisions were added to carry VAP-500 or VAP-1000 poison gas dispensers under the wings.<ref name=g100/>

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.<ref name=g101>Gunston, ''Tupolev Aircraft'', p. 101</ref> 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 {{convert|12.7|mm|sigfig=2|adj=on|sp=us}} [[Berezin UBT]] machine gun in each ShU [[barbette]] in each inner engine nacelle, on the underside of the wing covering the 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.<ref name=g101/> 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.<ref name=g75/>

===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.<ref name=g101/> Then, to compound the problem further, the production of AM-34FRNV engines ended in the second half of 1939. Only two or four Pe-8s were equipped with them. Eighteen of the aircraft produced by the end of 1940 were fitted with AM-35A engines.<ref>Gordon (2005), pp. 75–76</ref>

[[File:USSR stamp 989 Pe-8.jpg|thumb|upright=1.4|alt=Cancelled stamp illustrating a four-engined monoplane with a bomb between its landing gear. Text on the stamp reads "ПОЧТА СССР / 1 РУБ / Петляков-8 / Тяжелый бомбардировщик"|left|A Soviet stamp that reads "Post USSR / 1 [[Ruble|Rub]] / Petlyakov-8 / Heavy bomber"]]
In 1940, six aircraft without engines were fitted with [[Mikulin AM-35]]A engines, while VVS officials evaluated both the [[Charomskiy ACh-30]] and [[Charomskiy M-40]] [[aircraft Diesel engine]]s. At least nine Pe-8s were fitted with diesel engines in 1941, but neither the ACh-30 nor the M-40 were entirely satisfactory, despite greatly increasing the range of the aircraft. All surviving Pe-8s were re-engined with AM-35As by the end of 1941. Production continued slowly at Factory No. 124; most of the factory's resources were devoted to the higher-priority [[Petlyakov Pe-2]], a successful light bomber. At this time, most of these aircraft, re-designated as the Pe-8 after Petlyakov was killed in a Pe-2 crash on 12 January 1942, were built with out-of-production AM-35A engines.<ref>Gordon (2008), pp. 393–94</ref>

The 1,380-kW (1,850-hp) [[Shvetsov ASh-82]] radial engine was proposed as a replacement to alleviate the shortage of engines and this modification went into production in late 1942. The exhaust arrangements of the ASh-82 were not compatible with the gun turrets in the rear of the engine nacelles and the guns were removed, reducing the aircraft's defensive capability. At the end of 1943, the nose turret was deleted in favor of a manually operated ShKAS machine gun in a more streamlined nose.<ref name=g281>Gunston, ''Osprey Encyclopedia'', p. 281</ref> This version of the aircraft proved to have much the same range as the diesel-engined versions, but reliability was greatly improved. Production of the Pe-8s totaled 93.<ref>Gordon (2008), pp. 395, 398</ref>

The last Pe-8s were completed in 1944 as Pe-8ONs (''Osobovo Naznacheniya''—Special Mission) with Charomskiy ACh-30B engines and a [[Fillet (mechanics)|fillet]] at the base of the vertical stabilizer. These were special VIP transports with a seating capacity of twelve and a cargo capacity of {{convert|1200|kg|sp=us}}.<ref name=g76>Gordon (2005), p. 76</ref> Sources disagree as to whether the armament was removed and, if it was, whether partly or entirely.<ref name=g103>Gunston, ''Tupolev Aircraft'', p. 103</ref>