Design and developmentEdit
In the early 1930s, the German authorities started placing orders for new aircraft, initially training and utility aircraft. Heinkel, as one of the most experienced firms in the country, received contracts for a number of two-seat aircraft, including the He 45, He 46 and He 50. The company also worked on single-seat fighter designs, which culminated in the He 49 and later with the improved He 51 .
When the He 51 was tested in combat in the Spanish Civil War, it was shown that speed was far more important than maneuverability. The Luftwaffe took this lesson to heart, and started a series of design projects for much more modern aircraft. One of these projects, Rüstungsflugzeug IV, called for a day fighter with a top speed of 400 km/h (250 mph) at 6,000 m (19,500 ft) which it could maintain for 20 minutes out of a total endurance of 90 minutes. It also needed to be armed with at least three machine guns with 1,000 rpg, or one 20 mm cannon with 200 rounds. The specification required that the wing loading should be below 100 kg/m² - a way of defining the aircraft's ability to turn and climb. The priorities for the aircraft were level speed, climb speed, and then maneuverability in that order.In October 1933, Hermann Göring sent out a letter requesting aircraft companies consider the design of a "high speed courier aircraft" - a thinly veiled request for a new fighter. In May 1934, this was made official and the Technisches Amt sent out a request for a single-seat interceptor for the Rüstungsflugzeug IV role, this time under the guise of a "sports aircraft". The specification was first sent to the most experienced fighter designers, Heinkel, Arado, and Focke-Wulf. It was later sent to newcomer Bayerische Flugzeugwerke (Bavarian Aircraft Works or BFW) on the strength of their Bf 108 Taifun advanced sportsplane design. Each company was asked to build three prototypes for run-off testing. By spring 1935, both the Arado and Focke-Wulf aircraft were ready, the BFW arriving in March, and the He 112 in April.
Heinkel's design was created primarily by twin brothers Walter and Siegfried Günter, whose designs would dominate most of Heinkel's work. They started work on Projekt 1015 in late 1933 under the guise of the original courier aircraft, based around the BMW XV radial engine. Work was already under way when the official request went out on 2 May, and on 5 May the design was renamed the He 112.
The primary source of inspiration for the He 112 was their earlier He 70 Blitz ("Lightning") design. The Blitz was a single-engine, four-passenger version originally designed for use by Lufthansa, and it in turn was inspired by the famous Lockheed Model 9 Orion mail plane. Like many civilian designs of the time, the aircraft was pressed into military service and was used as a two-seat bomber (although mostly for reconnaissance) and served in this role in Spain. The Blitz introduced a number of new construction techniques to the Heinkel company; it was their first low-wing monoplane, their first with retractable landing gear, their first all-metal monocoque design, and its elliptical, reverse-gull wing planform would be seen on a number of later projects. The Blitz could almost meet the new fighter requirements itself, so it is not surprising that the Günters would choose to work with the existing design as much as possible.
In many ways, the resulting He 112 design was a scaled-down He 70. Like the He 70, the He 112 was constructed entirely of metal, using a two-spar wing and a monocoque fuselage with flush-head rivets. The landing gear retracted outward from the low point of the wing's gull-bend, which resulted in a fairly wide 9 m (30 ft) track, giving the aircraft excellent ground handling. Its only features from an older era were its open cockpit and fuselage spine behind the headrest, which were included to provide excellent vision and make the biplane-trained pilots feel more comfortable.