The Heinkel He 274 was a German heavy bomber design developed during World War II, purpose-designed for high-altitude bombing with pressurized crew accommodation.
Due to the Allied advance through North-west Europe, the prototypes were abandoned at the French factory where they were being built. They were completed after the war by the French and used for high-altitude research.
== He 177 ancestry ==
On 17 November 1938, the owner of the Heinkel aviation firm, Ernst Heinkel, requested permission from the RLM that two of the requested eight prototype airframes for the nascent He 177 heavy bomber project, specifically the V3 and V4 airframes, be set aside for a trial installation of four separate Junkers Jumo 211 powerplants.〔Griehl & Dressel 1998, p. 14〕 Heinkel had foreseen that an individually engined version of his bomber would someday be preferred, quite unlike the requested fitment of the coupled pairs of Daimler-Benz DB 601 inverted V12 engines, each known as a DB 606 — weighing some 1.5 tonnes apiece — which ended up being fitted to all of the eight He 177 V-series prototypes at the request of the RLM, and the Luftwaffe High Command, with the concerned government agencies citing the desire for a dive-bombing capability to be present even with a heavy-bomber-sized offensive warplane, something Ernst Heinkel vehemently disagreed with.
By April 1939, interest in developing a high-altitude version of the He 177 had arisen, and on April 27, 1939, the first proposal for such an aircraft was presented to Heinkel by his firm's engineering staff.〔Griehl & Dressel 1998, p. 176〕 The aircraft was intended to have a reduced crew manifest of three people, with a fully pressurized nose compartment for the pilot and bombardier/forward gunner, and separate pressurized tail gun emplacement. The result, in December 1940, was the specification for the He 177A-2 high altitude bomber design, with a four-person crew manifest (pilot, bombardier, forward gunner and tail gunner) in the two specified pressurized compartments, and powered by the regular A-series pair of DB 606 coupled engines. The defensive armament had been reduced to a trio of ''Ferngesteuert-Lafette'' FL 81Z remote gun turrets, each with a twin-barrel MG 81 armament installation each in an upper nose mount, forward dorsal and (as part of the ''Bola'' casemate-style gondola under the nose) forward ventral location each, and a single MG 131 machine gun in an He 177A-1-style, pressurized manned flexible tailgun emplacement.〔Griehl & Dressel 1998, p.227〕 The A-2 version had even been considered for a pioneering in-flight refueling capability, possibly using Ju 290 maritime patrol aircraft as the tankers - with such capability, the range of the A-2 would have been extendable to some 9,500 km (5,900 mi) of total flight distance.
The Heinkel firm had been working on practical cockpit pressurization methods and hardware for both the A-2, and slightly later A-4 versions (identical to the A-2, except for the fitting of a pair of the later DB 610 coupled engine "power systems") from 1940 through the late summer of 1941, when the DB 610-powered A-4's pressurized cockpit in provisional form, almost identical in external appearance to the standard "Cabin 3" He 177A-series production cockpit, was ready for tests and development.
By October 1941, a more developed "He 177H" specification for a high-altitude Heinkel-designed heavy bomber〔Griehl & Dressel 1998, p. 177〕 had emerged from the proposed A-2 and A-4 coupled-engine designs, with the intent of carrying a 2,000 kg (4,410 lb) bombload over a maximum range of some 3,000 km (1,895 mi), and accepted by the RLM for the first time since the rejection of Dr. Heinkel's initial November 1938 request for two of the early He 177 V-series prototype airframes to get them, an individual four-engine installation was being considered for any He 177-based bomber airframe, with a quartet of either BMW 801 or DB 603 unitized-installation engines — the DB 603 powerplants being unitized in a Heinkel factory-specific design also used for the He 219 — among the choices of powerplants being specified, with the same sort of reduced-armament defensive weapon format as the A-2 and A-4 were intended to have.
In conjunction with his request for help from then-''Generalmajor'' Eccard Freiherr von Gablenz in May 1942 concerning the suitability of aircraft for the Amerika Bomber contract competition as that proposal first appeared, ''Generalfeldmarschall'' Erhard Milch also received von Gablenz's opinion on both the He 177 and its He 274 development, with von Gablenz stating that neither of the then-existing Heinkel "heavy bomber" designs had anywhere near the range to conduct a mission approaching the demands of the new contract.
A pair of the early He 177A-0 pre-production prototypes were redesignated the He 177 V10 and V11 for the purposes of high-altitude trials, and were to be the first to test the A-4 pattern pressurized cockpit design at altitude, but only the V11 was actively used for the needed research, and managed to achieve an altitude of 9,200 m (30,200 ft) with complete success on August 9, 1943, with further tests continuing through October of that year, before both the V10 and V11 were grounded in April 1944.〔Griehl & Dressel 1998, p.179〕
In February 1943, the same month during which the RLM first mentioned any official status for design work on an entirely separate, Heinkel He 277-named heavy bomber design to be paid to Heinkel's engineering shops by them, and becoming the Heinkel firm's ''Amerika Bomber'' contract contender, any further work on the coupled-engined He 177A-2 and A-4 designs was halted by order of the RLM, as the four-engined He 177H high-altitude design had gained in importance from that time, evidenced by ''Reichsmarschall'' Hermann Göring's derisive "welded-together engines" complaints in August 1942, regarding the He 177 A-series unending engine problems from the choice of the DB 606 and 610 "power systems" for the A-series operational aircraft. This resulted in a trio of parallel programs under simultaneous development by the Heinkel engineering shops for four-engined heavy bomber designs, from the February 1943 date through to April 20, 1944.
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