Dornier Do 17/Do 215

In 1931 Dornier set about designing the Do F twin-engined bomber. It was first flown on 7th May 1932 in Altenrhein. Its fuselage was of all-metal, stressed-skin construction. The wings were fitted with metal spars and ribs, and covered partially with fabric. The aircraft was powered by two 600 hp Bristol Jupiter radial engines, produced under licence by Siemens. The Do 11 (which was the official designation for the Do F) was included in the 1932-1938 expansion plan for the German Air Force. The production of the Do 11 and Militär-Wal 33 flying boat was undertaken by the Dornier company (which around that time was re-named Dornier-Werke GmbH) in 1933.
When, in January 1933, the National Socialists seized power, the German Air Force entered a period of rapid expansion. On 5th May 1933 the State Ministry of Aviation (Reichsluftfahrtministerium or RLM) was formed, with Hermann Göring at its head, and Erhard Milch as the State Secretary for Aviation. The RLM quickly devised a new expansion programme, which aimed at creating a fleet of 400 bombers – the Luftwaffe’s main striking force – by the end of 1935.
A technical specification for a Kampfzerstörer, a fast assault/bomber aircraft, was created in Obstlt. Wilhelm Wimmer’s Waffenprüfwesen (weapon test division), which was part of the Heereswaffenamt (Ordnance Department) of the Reichswehrministerium (Reich Defence Ministry), as early as July 1932. At that time Germany was still under close scrutiny from the international community. Hence, Gen. Lt. von Vollard-Bockelburg, then at the head of the Heereswaffenamt, concealed the aircraft’s intended role by issuing specifications for a Schnellverkehrsflugzeug für die DLH (fast communication aircraft for the DLH)3. However, there was no doubt that the design was to be a combat aircraft. Its civilian use was of minor importance, and allowed only if the aircraft could be quickly and inexpensively re-converted to its military version. Invitations to tender were sent to three companies: Dornier, Junkers and Heinkel. The design bureaus eagerly accepted the challenge to create an aircraft that would meet the demands stated by the Army. Thus, three bombers were designed: the Dornier Do 17, the Junkers Ju 86 and the Heinkel He 111.

dornier 4

Of the three producers, Dornier seemed the least concerned about the aircraft’s secondary use as a civilian machine. His design, which incorporated all the latest achievements in aerodynamics, featured an unusually slim, long and narrow fuselage, which could accommodate two bomb bays, but had hardly enough room for its six passengers. In March 1933 Dornier prepared a full-scale wooden mock-up. On 17th March 1933 it was demonstrated to representatives of the Reichskommissariat für die Luftfahrt (Reich Commissariat for Aviation). On 5th May 1933 the newly established RLM took over the responsibilities of the Reichskommissariat für die Luftfahrt. Immediate control over the development of new aircraft designs was placed with the Abteilung Technik (technical division) of the Allgemeines Luftamt, the civilian department of the RLM, under Obstlt. Wilhelm Wimmer.
On 23rd March 1933 Erhard Milch, the Staatssekretär der Luftfahrt of the RLM, placed an order with Dornier for two aircraft designated Do 17, one in military configuration, and the other for Lufthansa. It was stressed that the civilian version was to be easily modified for military use. The first prototype of the military version was designated Do 17 C, W.Nr. 256 (later Do 17 V1). The prototype of the civilian version was initially designated Do 17 A, W.Nr. 257 (later Do 17 V2). Both aircraft were to be powered by BMW VI (Do 17 C) or BMW VI 6.0 (Do 17 A) inline engines. On 2nd October 1933 Dornier proposed a third prototype powered by Hispano-Suiza 12Ybrs engines, which was to serve as a ‘fast airliner’. On 4th November 1933 the RLM signed the contract, and the aircraft received the designation Do 17 D.
The Do 17 was a markedly advanced design for the first half of the thirties. The airframe was composed of four main sub-parts: the fuselage front and rear sections, the wings and empennage. Its fuselage was a metal monocoque of built-up frames and intermediate stiffeners notched to receive channel-section stringers, and its wing was a two-spar trapezoidal structure, the spar booms being thick duralumin extrusions of asymmetrical section, the girder-type spar bracing utilising duralumin members of broad channel section, from which the main ribs were built up, and the intermediate ribs had a tubular bracing. Wing skinning was flush-riveted light metal, apart from the undersurface of the wing between the spars, for which fabric covering was employed.
The Do 17 C prototype was designed to feature the classic, single tailfin configuration, whilst the Do 17 A prototype was to be equipped with twin tailfins, successfully used by Dornier on his Do 23 bomber. All fuel was housed in the wing centre section, between the engine nacelles and the fuselage, and the main undercarriage members retracted aft hydraulically and mechanically into the tails of the engine nacelles.