Dornier Do 217
The Dornier Do 217 was a bomber used by the German Luftwaffe during World War II. It was a more powerful development of the Dornier Do 17, known as the Fliegender Bleistift. Designed in 1937–1938 as a heavy bomber but not meant to be capable of the longer-range missions envisioned for the larger Heinkel He 177, the Do 217 design was refined during 1939 and production began in late 1940. It entered service in early 1941 and by the beginning of 1942 was available in significant numbers.
The Dornier Do 217 had a much larger bomb load and a much greater range than the Do 17. In later variants, dive bombing and maritime strike capabilities using glide bombs were experimented with, considerable success being achieved. Early Do 217 variants were more powerful than the contemporary Heinkel He 111 and Junkers Ju 88, having a greater speed, range and bomb load. Owing to this it was called a heavy bomber rather than a medium bomber. The Do 217 served on all fronts in all roles. On the Eastern Front and Western Front it was used as a strategic bomber, torpedo bomber and reconnaissance aircraft. It was also used for tactical operations, either direct ground assault or anti-shipping strikes during the Battle of the Atlantic and Battle of Normandy. The Do 217 was also converted to become a night fighter and saw considerable action in the Defence of the Reich campaign until late in the war.
The type also served in anti-shipping units in the Mediterranean, attacking Allied convoys and naval units during the Battle of the Mediterranean. In 1943, the Do 217 was the first aircraft to deploy precision-guided munitions in combat, when Fritz X radio-guided bombs sank the Italian battleship Roma in the Mediterranean. After the end of the war, at least one Dornier Do 217 continued in military operational service with the Swiss Air Force until 1946.
Development and design
Early designs and marine plans
At the beginning of 1938, Dornier issued manufacturing specification No. 1323, recognising the need for a twin-engine bomber or long-range reconnaissance aircraft powered by Daimler-Benz DB 601B engines. In February 1938 the Reichsluftfahrtministerium authorized a testing program.Dornier worked on a version of the Do 17M with the all round vision cockpit of the Do 17Z and a fuselage with a large bomb bay capable of holding a maximum of two and ten bombs. For reconnaissance an Rb 50/30 movie camera was fitted ahead of the front spar of the wing, and an Rb 20/30 was mounted in the second bomb bay. Jettisonable fuel tanks were carried in the forward bomb bay. For smoke-laying, the aircraft could be fitted with two Type S200 smoke generators. Dornier also envisaged the Do 217 as a naval dive bomber, in which case it was to be fitted with twin floats. In April and May 1938, the Do 217 WV1 and WV2 prototypes were produced.
The wing span was to be one metre greater than that of the Dornier Do 17, giving an overall span of. Under the wing a retractable diving air brake was to be installed. To power the aircraft the Dornier office at Manzell favoured two DB 601B engines, which could generate 1175 PS for take off. The Jumo 211, Bramo 329 and BMW 139 were also considered. Whichever power plant was selected, the RLM expected the aircraft to have a maximum speed of and weigh fully loaded.
On 5 June 1938 Dornier's overview of its design submitted to the Technical Bureau highlighted some structural differences from the Do 17. In particular, the proposed increase in the bomb load to had to have been a vital factor in the design's acceptance. The fuselage was to be not only bigger but also stronger.
The RLM also had other requirements for Dornier to satisfy. Since 1933 the Kriegsmarine had pressed for the formation of a Naval Air Arm. In January 1938 the Naval Air Arm Inspectorate of the Luftwaffe presented its requirements for a multi-role twin-engined all-metal aircraft which could also conduct maritime operations. On 5 February 1938 it was agreed with the General Staff. The ineffectiveness of horizontal bombing of ship targets had already been noted. At the Erprobungsstelle Travemünde military aviation test centre at Greifswald, training units together with a few naval air units practiced bombing the ship Zähringen with concrete bombs. The results were a two percent hit rate. Junkers Ju 87 dive bombers repeated the assault with a 40% hit rate. The superior accuracy of dive bombing was clearly demonstrated. The Luftwaffe also wanted a machine that could operate as a fighter aircraft to combat enemy aircraft. Essentially they wanted a "sea Stuka". The aircraft was to have floats and a range of and a maximum speed of.
Dornier set about designing a floatplane. For hitting targets in the air and sea surface, four automatic weapons would be fitted in the nose. The armament would consist of two MG 17 machine guns and two MG 204 20mm autocannon as part of the Dornier Do P.85 project. Heavy weapons consisted of one 500 kg or two 250 kg bombs for assaulting enemy warships. Dornier faced competition from Heinkel and Junkers who were developing the Junkers Ju 88 and Heinkel He 115.
Instead of MG 204s, MG 151 or MG FF was installed instead. Defensive armament was to consist of MG 15s to cover the aft approach. In addition the payload could be modified to one and eight bombs or even two SC 500 bombs. The floats had a volume of and were broken into compartments for safety reasons. Each float was to contain a fuel tank with a capacity of. The tail was to consist of the same twin stabilizer configuration as the Do 217, although a single fin was planned.
For dive bombing capability a dive brake was installed underneath the wing. The power plants were to reflect the speed requirements. It was envisaged as having two DB 601G engines, generating, or two Jumo 211s. The fuel tanks were located in the wing and fuselage which had a capacity for of fuel and of oil. At full weight the Dornier would reach and its effective range was expected to be. Its optimum range at an average cruising speed of at an altitude of, was. The specifications were dated 8 March 1938.
The Do 217 lost out in the naval aircraft race to the Ju 88 and Blohm & Voss Ha 140, as the Luftwaffe favoured these designs owing to the Do 217 failing to live up to the specifications given.
Although specifically ordered to cease development of the naval version of the Do 217, Dornier unofficially pursued the project and produced the Do 217W V1 and W V2 prototypes.
By the summer, 1940 the Luftwaffe had been using the Dornier Do 18, Heinkel He 115, Heinkel He 59, Heinkel He 111 and Junkers Ju 88 in maritime operations in the Baltic. At this point, the Ju 88 and He 111 equipped units were ordered to cease providing maritime support en masse. Instead, the Luftwaffe returned to the idea of the Do 217 and its floatplane version as a specialized naval attack aircraft. At the same time more plans were in place to produce extremely long-range aircraft. It is possible that the data sheet which Dornier gave the designation Do 217G was a part of that project. Unlike the Sea Stuka, a floatplane, the G was to carry an MG 151 mounted in the nose and three MG 15s fitted for defence. The G was expected to reach. It was still designed for a crew of four and equipped with sprung floats which would allow the aircraft to land in rough open seas. The G could also carry the entire variation of the E-1 bomb load it could carry a load twice that of the Do P.85 aircraft. However, the Do 217 E-1s performance was favoured. Nevertheless, the Gs design features figured and influenced the E-4 which went into production as the aircraft that was envisaged to be the backbone of the Luftwaffe's bomber fleet in the Battle of the Atlantic.
Focus on a heavy bomber
At the end of August 1938 arguments against the floatplane version were made and proposing a land based aircraft to serve as a torpedo bomber, with more potential applications, were accepted. At the beginning of January 1939 the RLM stopped all work on the marine dive bomber version, as its estimated performance was not adequate.On 8 July 1939 Dornier issued a manufacturing specification for a glide bomb-deploying version for full maritime use. It was to be equipped with unitized BMW 801 engines. The Do 217 E had a new nose and the nose, cockpit rear, and ventral positions carried one MG 15 each. It was to carry a maximum bomb load of two SC 500 and two SC 250 bombs. It was also possible to carry an aerial mine or torpedo, for which the bomb bay had been substantially extended rearwards, nearly 70% longer than that of the earlier Do 17Z.
A "clamshell"-like dive brake was fitted aft of the tail, with rear-hinged single dorsal and ventral panels. These features increased the design's weight to. Dornier had intended the speed to be in the region of.
Superficially a bigger Dornier Do 215, and initially powered by the same engines, the Do 217 was actually considerably larger and totally different in both structural and aerodynamic design. The first prototype flew on 4 October 1938, but crashed seven days later during a single-engine flying test. The aircraft had been piloted by Rolf Koeppe, a flight commander at the central Erprobungsstelle facility at Rechlin. A Dornier mechanic, Eugen Bausenhart was also on board. It was found to be underpowered and was not manoeuvrable when compared with contemporary bombers. Instability was a problem at first, but modifications such as fixed Handley-Page leading edge slots along the leading edges of the vertical stabilizers helped to improve flight stability.
The second prototype flew on 5 November 1938. After arriving at Friedrichshafen in June 1939, further evaluations were scheduled to take place. Plans were made to install unitized Daimler-Benz DB 603 engines to enable the aircraft for high-altitude reconnaissance. This meant the fitting of a pressurized cabin. When Daimler-Benz failed to supply the engines, development came to a standstill. On 29 October the RLM ordered the aircraft to be scrapped, or a new use found for it.
A third prototype flew on 25 February 1939 with Jumo 211A engines in place of the DB 601s. On 15 August 1939 and 23 January 1940, the aircraft was flown to Rechlin, where it was tested in night flying trials. A number of the flights were to assess the performance of the new Siemens navigation aids under development. At the same time, Dornier also carried out fuel jettisoning and drop tank trials using the standardized 900 L capacity, vertical-finned drop tanks pioneered by the Bf 110D extended-range heavy fighter. As with the Do 17, the test team tried several tail configurations with the Do 217 V3. single, double and triangular-planform assemblies were tried. These designs were used in the Do 217 M-3, M-9 and Dornier Do 317.
The same units were used on the fourth prototype V4 which flew in April 1939 at Friedrichshafen and Rechlin. The Jumo proved to be superior, and the designers deemed them to be essential if the desired performance was to be achieved. In February 1941, the V4 began trials with the dive brake which was installed in the tail. This was to satisfy a demand for the Do 217 to conduct dive bombing missions. It also was fitted with a brake parachute to test the ability of the Dornier to conduct short landings. The parachute brake was also considered in use as a dive brake. The V5 prototype was fitted with them and flew in June 1939. Later it was retested with DB 601s and was the third of six aircraft given the official designation Do 217 A-0. The Jumo 211 B-1 was used in the V5 prototype. But in September 1939 the water pump and entire cooling system failed. On 28 April 1940 the DB 601 A-1s were fitted.
The V6 prototype was powered by Jumo 211B engines, but was also tested with DB 601s. The V7 was tested with BMW 139 engines, but as these had been abandoned for use in the Fw 190 fighter as early as 1939, use of the unpopular BMW 139 powerplants was never taken beyond the prototype stage. The V8 was given BMW 801 engines, which became the fixture for the entire E series. The Do 217A and C series were only built in small numbers. Owing to this the following D and F types never advanced beyond the design stage.
There was a desire for the Do 217 to be capable of performing dive bombing, so it was therefore fitted with the aforementioned, tail-mounted dive brakes, with dorsal and ventral panels that were hinged, "clamshell"-fashion, at the extreme rear of the tail extension they emerged from. This could not be made to function adequately in the early models however, and was omitted until the Do 217 E-2 entered service. When this mark reached service, use of the dive brake was found to sometimes overstrain the rear fuselage, so it was often removed.
The production specifications were ratified on 8 July 1939, with the ultimate goal of the Do 217 having the capability of flying maritime and land operations armed with glide bombs. The four-seat aircraft was adaptable to both land and maritime operations wherein the tactical emphasis was on bombing from a 50-degree dive angle, and it had a maximum speed of. In contrast with earlier specifications for a modified version of the Do 17M, the proposed Do 217E had a new nose section design in which the A-Stand position was armed with a MG 15 machine gun. Additional MG 15s were to be located in the B and C-Stand gun emplacements. The design teams configured the bomb bay to carry two SC 500 and 250 bombs or four SC 250 bomb loads. In addition a LMB II aerial mine, or an F5 Torpedo could be loaded. Instead of the dive brakes being installed under the wings as on the R variant, it was placed on the tail of the aircraft. The design was tested in the E-1 and became the blueprint for all subsequent sub-variants. The E-1 carried strengthened wing and tail structures to deal with the upgraded armament, which increased the aircraft's weight.