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 Northwest 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.
Background
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. 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 27 April 1939, the first proposal for such an aircraft was presented to Heinkel by his firm's engineering staff. The aircraft was intended to have a reduced crew of three, 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 (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. 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 in-flight refueling, 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 engines) 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 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 Generalmajor Eccard Freiherr von Gablenz in May 1942, concerning the suitability of aircraft for the Amerikabomber contract competition as that proposal first appeared, Generalfeldmarschall Erhard Milch also received von Gablenz's opinion on the He 177 and its He 274 development, with von Gablenz stating that neither of these Heinkel "heavy bomber" designs had anywhere near the range to conduct a mission approaching the demands of the new contract.