Wing

The Do 217 was a shoulder-wing cantilever monoplane. Its two-spar wing was built in three sections: the centre section, incorporating part of the fuselage, and two outer wing sections with very little taper on the leading and trailing edges, leading out to a pair of broad, semi-circular wing tips. The stress bearing skin was riveted to spars and ribs. Owing to its future use as a dive-bomber, stressed skin construction was employed with the use of Z-section frames and stringers. Slotted ailerons were fitted to the outer wing sections. The inner split flaps were electrically operated and had a maximum flap angle of 55°. The ailerons were linked to the flap system to permit partial operation as flaperons, meaning if the flaps were lowered the ailerons drooped down. The design of the rear and front spar attachments revealed the flanges of the spar were flushed with the wing surface making the most efficient structure. The outer wing leading edges were double-skinned. In the wing space, a hot air feed was fitted, using heat pumped through lagged pipes from the engines to warm up and de-ice the wings. The ducts were located just forward of the front spar flanges and in between the main spars where they could escape into the wing. The hot air could escape through the apertures at the aileron hinges. A diversion pipe was also installed in the engine nacelle, which could shut down the hot air flow to the ducts and divert the air out through bottom end of the nacelle if de-icing was not required. The fuel and oil tanks were located in the wing and centre section. The two outer fuel tanks were located next to the outer side of the engine nacelle. The outermost fuel tank contained a 160 litres (35 imp gal) fuel capacity, while the tank closest to the engine could accommodate 44 imperial gallons (200 l) of oil. Both were sandwiched in between the main and rear main spar. Between the fuselage and inner side of the engine nacelle, the 795 litres (175 imp gal) main tanks were located. In the centre section a 1,050 litres (230 imp gal) fuel tank was installed forward of the bomb bay.

Fuselage

The fuselage was an all-metal structure built in three main sections. These consisted of the nose section, which accommodated the crew; the section built integrally with the wing centre section; and the rear fuselage. The main structure was built out of the standard stringer and former skeleton design, to which stressed skin was riveted. The centre and rear sections were divided in the horizontal plane to within a couple of metres of the tail. The lower half of the fuselage contained the bomb cells, while the remainder and upper parts contained the bracing frames which supported and kept the bomb load secure. In the lower half of the fuselage the bomb bay stretched past the trailing edge wing roots to nearly a quarter of the way down the rear fuselage. Extra doors were added for torpedo operations against Naval targets. The extreme rear of the fuselage contained stowage space for anti-shipping weapons. In the upper fuselage, directly above the forward bomb cells in the centre section, just aft of the cockpit, the 242 gallon fuel tank was located. Attached to this was a fuel jettisoning pipe which was fitted to the roof of the fuselage and ran along it to the tail of the aircraft. It declined and exited the fuselage behind the tail wheel to allow fuel dumping in an emergency. Above the two rear bomb cells in the centre section, a dinghy stowage place was created. To protect its very vulnerable structure against enemy fire, it was placed in a heavily armoured case. Either side of the dinghy, aft of the 242 imperial gallons (1,100 l) fuel tank, and directly above the rear bomb bay, were the oxygen bottles.

In the extreme tail unit, the tail wheel was retractable and had its own folding doors. The tip or end of t he fuselage could be removed to allow quick access for mechanics to enter the tail section and repair the jack mechanism which controlled the tail plane incidence and pivot. The tail incidence was automatically changed when the landing flaps were lowered, or it could be adjusted manually. This tip would be replaced by dive brakes in the event the Dornier was required to attack precision targets. The horizontal stabilizer surfaces were conventional, as part of a twin tail empennage with "endplate" vertical fin/rudder units, as on the previous production Do 17 airframes. The rudder contained a balance tab while the elevators contained an elevator balance tab and an automatic dive pull-out tab, in the event of a dive-bombing mission. The stabilisers were fitted with fixed slats, with the trailing edges of the slats positioned inside of the fins. The rudders had very narrow horn balances (the amount of rudder or active control surface forward of the rudder-stabilizer hinge) which allowed for better balance, and the trimming tabs extended the full length of the stabilizers trailing edges. The stabilizers also had the Handley-Page leading edge slots installed on the inboard side.

Powerplants

All the Do 217E variants were fitted with unitized, BMW 801A air-cooled 14-cylinder radial aircraft engine built by BMW. The early E-series variants were meant to be fitted with a BMW 801B engine design for portside use to employ counter-rotating propellers as a few other multi-engined Luftwaffe aircraft were designed to have (as with the He 177A) but this was never accomplished. The BMW 801 radials were mounted on welded steel tube mountings at the extremities of the wing centre section. The oil coolers were integral to the front lower section of the BMW-designed cowlings, as used in all twin and multi-engined aircraft that used BMW 801 radials for power. The machine had two oil and five fuel tanks to "feed" the engines, and they were protected with rubber and self-sealing coverings. In an emergency, CO2 could be released into the tanks to extinguish fires. The engines had three-blade, light alloy VDM propeller units installed. Flame dampers were also fitted in the form of "fish tail" exhaust pipes. Such a configuration gave a speed of 470 kilometres per hour (250 kn) (282 mph) at sea level and 530 kilometres per hour (290 kn) (318 mph) at 6,700 metres (22,000 ft). It enabled a service ceiling of 7,300 metres (24,000 ft) when fully loaded and 8,200 metres (26,900 ft) when lightly loaded. The Do 217's range was a much more impressive 3,900 kilometres (2,400 mi), (compared with the other German bomber types). The engines and their supporting struts were positioned well forward of the leading edge allowing plenty of room for the undercarriage and other components. In the upper forward part of the nacelle the de-icing tank was located. The undercarriage was the main structure in this part of the aircraft. Each main unit comprised two oleo legs and a single wheel. It was retracted electrically via the crew in the cockpit. The adoption by the Luftwaffe midway through the war, of a general system of unitized powerplant installations for twin and multi-engined combat aircraft incorporating as many of the engine's auxiliary components (radiator and oil cooler, and all fluid and mechanical connections) into a single, easily interchangeable unitized "bolt-on" package, known as the Kraftei (power-egg) concept, was being widely adopted by the time of the Do 217's initial frontline appearance. The Junkers Jumo 211 was one of the first engines to be unitized as seen on the Ju 88A, with the BMW 801 radial and Daimler-Benz DB 603 following not long afterwards, as both the BMW radial and the DB 603 inline engines were to be used in such a "unitized" format to power the frontline models of the Do 217.

Cockpit

The Do 217 usually carried a crew of four. Included were a pilot, an observer/bomb aimer/forward gunner, dorsal gunner/radio operator and a flight engineer/ventral gunner. As with the Dornier Do 17, the crew were positioned in the cockpit cabin forward of the engines and leading edge. The pilot sat on the port side, with a spectacle-type control column mounted on a swinging arm centred in the instrument panel. The entire arm could be swung 180 degrees to starboard, placing the yoke in front of the bombardier, in case of emergencies. In the later Do 217K with stepless windscreen, the centre pivot was mounted on a rocking control arm or pedestal, rather than a sliding piston, since the instrument panel itself was eliminated (replaced with smaller panels holding the gauges, hung from the edge of the fuselage wall and from the glazing frames above the yoke, respectively.) The bomb aimer sat on his immediate right, slightly below and behind. In combat he could move forward into the nose and operate the bomb release gear or aim the forward machine gun (or 20 mm cannon). On the right side of the nose, the bomb aimer's window penetrates the cockpit and is visible as a bulge on the exterior. Back-to-back with the bombardiers seat, the flight engineer/ventral gunner sat on the port side, his seat facing to the rear. The seat of the ventral gunner/flight engineer was next to the radio operator facing forward, behind the bomb aimer. During operations the ventral gunner/engineer would lie on his stomach facing aft, his post as a gunner taking immediate and first priority. The radio-operator/dorsal gunner sat in a pivoting seat in the extreme rear, above the ventral gun position. His head was inside the B-Stand gun position, and the instruments mounted in a semi-circle around his torso level. The pilot had a curved shield of Armour plating, 8.5 mm thick, placed behind his seat. His seat had a further 5mm (​1⁄4 in) of armour and another 5mm (​1⁄4 in) plate above his head, installed in the top of the cockpit roof. The dorsal gunner was also protected by armour plating. The Funkgerät or FuG radio communication device compartments were located in the extreme rear of the cockpit, near the leading edge. The automatic pilot panel was located next to the FuG boxes. In the right-rear of the cockpit the piping that was also attached to the de-icing ducts in the wings also entered the cockpit, as part of a single heating system to emit warm air to heat the cabin if needed. The FuG X, 16, navigational direction finder PeilG V direction finder (PeilG – Peilgerät) and the FuG 25 IFF and FuBI 2 blind landing devices were used in the E-2.

Armament

The defensive armament consisted of an A-Stand (forward firing machine gun position) in the nose with a MG 15 machine gun. In the C-Stand (lower rear gun emplacement) at the rear end of the undernose Bola gondola – a standard feature on many German twin-engined bombers – and a B-Stand position (rear-upper gun post) at the rear of the cockpit glazing, the crew were provided with MG 15 (E-1) or MG 131 machine guns (E-2). In the side of the cockpit, two MG 15s were mounted (one on each side) on bearings. As well as the manual machine gun positions, the E-2 was equipped with a Drehlafette DL 131 rotating turret armed with a 13 mm machine gun. In some instances, a moveable 20 mm cannon was fitted in the nose and a fixed 15 mm weapon was installed in the floor of the nose. The weapons could be controlled by the pilot, via a firing button located on the yoke. To assist in acquiring his target, a Revi C12/C was installed in the cockpit.

The maximum permissible bomb load of the E-series without sacrificing fuel load was 3,500 kilograms (7,700 lb) of bombs of which a maximum of 3,000 kg could be carried internally. To have the maximum load of 4,000 kg, part of the 1,050 litres (230 imp gal) fuselage fuel tank had to be sacrificed. If long distance operations had to be flown, drop tanks could be fitted under the wings, which affected speed. Beside the bomb load a LT F5 Torpedo could be carried in its long bomb bay, as well as three aerial mines (the E-1 did not carry the mine load of the E-2).

Sources:
Gunston, Bill - The Encyclodepia of the Worlds Combat aircraft, 1976, Chartwell Books, Inc., New York
Brown, Eric, Captain - Wings of the Luftwaffe, 1979, Airlife Publishing Ltd., Shrewsbury
Gunston, Bill & Wood, Tony - Hitler's Luftwaffe, 1977, Salamander Books Ltd., London
Donald, David - The Complete Encyclopedia Of World Aircraft, 1997, Brown Packaging Books Ltd., London
Wikipedia - Dornier Do 217