A natural twin-engined outgrowth of the He 70, the first He 111 was a graceful machine with elliptical wings and tail, secretly flown as a bomber but revealed to the world a year later as a civil airliner. Powered by 660 hp BMW VI engines, it had typical armament of three manually aimed machine guns but the useful bomb load of 2,200 lb (1,000 kg) stowed nose-up in eight cells in the centre fuselage. In 1937 a number of generally similar machines secretly flew photo-reconnaisance missions over Britain, France and the Soviet Union, in the guise of airliners of Deutsche Luft Hansa. In the same year the He 111B-1 came into Luftwaffe service, with two 880 hp Daimler-Benz DB 600C engines, while a vast new factory was built at Oranienburg solely to make later versions. In February 1937 operations began with the Legion Kondor in Spain, with considerable success, flight performance being improved in the B-2 by 950 hp DB 600CG engines which were retained in the C series. The D was faster, with the 1,000 hp Jumo 211A-1, also used in the He 111 F in which a new straight-edged wing was introduced. To a considerable degree the success of the early elliptical-winged He 111 bombers in Spain misled the Luftwaffe into considering that nothing could withstand the onslaught of their huge fleets of medium bombers.
These aircraft, namely the trim Dornier Do 17, the broad-winged He 111 and the high-performance Junkers Ju 88, were all extremely advanced by the standards of the mid-1930s when they were designed. They were faster than the single-seat fighters of that era and, so the argument went, therefore did not need much defensive armament. So the three machine guns carried by the first He 111 bombers in 1936 stayed unchanged until, in the Battle of Britain, the He 111 was hacked down with ease, its only defense being its toughness and ability to return to base after being shot to pieces. The inevitable result was that more and more defensive guns were added, needing a fifth or even a sixth crew member. Coupled with incessant growth in equipment and armour the result was deteriorating performance, so that the record-breaker of 1936-38 became the lumbering sitting duck of 1942-45. Yet the He 111 was built in ever-greater numbers, virtually all the later sub-types being members of the prolific H-series. Variations were legion, including versions with large barrage-balloon deflectors, several kinds of missiles (including a V-1 tucked under the left wing-root), while a few were completed as saboteur transports. The most numerous version was the H-6, and the extraordinary He 111Z (Zwilling) glider tug of 1942 consisted of two H-6s joined by a common centre wing carrying a fifth engine. Right to the end of the war the RLM and German industry failed to find a replacement for the old "Spaten" (spade), and the total produced in Germany and Romania was at least 6,086 and possibly more than 7,000. Merlin engined C.2111 versions continued in production in Spain until 1956.