Do 217 A-0:

Pre-production series with two 1,085 hp DB 601B inline engines, used for reconnaissance missions. Only eight aircraft were built.
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Do 217 C-0:

Pre-production bomber, DB 601B engines, increased defensive armament. Only four built.
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Do 217 E-0:

Pre-production bomber with deepened fuselage and powered by two 1,560 PS 1,147 kilowatts (1,538 hp) BMW 801A engines.
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Do 217 E-1:

Production bomber with five 7.92 mm (.312 in) MG 15 machine guns and one 15 mm MG 151 cannon for defence.
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Do 217 E-2:

Bomber with dive bombing capabilities, with three 7.92 mm MG 15s, two 13 mm (.51 in) MG 131 machine guns, one 15 mm MG 151, and a 4,000 kilograms (8,800 lb) bombload.
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Do 217 E-3:

Level bomber, seven 7.92 mm (.312 in) MG 15s and one forward-firing 20 mm MG FF cannon.
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Do 217 E-4:

BMW 801L engines.
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Do 217 E-5:

E-4 with extended wingspan, modified on production line with added Kehl radio control gear to launch Henschel Hs 293 missiles.
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Do 217G:

In January 1940 Dornier produced a data sheet for a Do 217G with BMW 801 engines with the same airframe as a Do 217E-1. None were built

Do 217H:

Conversion of a Do 217E with DB 601 engines fitted with experimental turbo-superchargers.
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Do 217J:

Night fighter based on the Do 217E. Solid nose with four 7.92 mm MG 17 machine guns and four 20 mm MG FFs.
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Do 217 J-1:

Night intruder version.
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Do 217 J-2:

Dedicated night fighter. Bomb bays removed.
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Do 217K:

Bomber with redesigned forward fuselage with stepped windscreen eliminated, a common World War II German bomber cockpit design concept adopted just before the start of World War II initially on earlier Heinkel bombers (He 111P and 177A). Two 1,560 PS BMW 801L radial engines.
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Do 217 K-1:

Standard bomber version.
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Do 217 K-2:

Extended wingspan with Kehl radio gear to carry Fritz X bombs on underwing racks.
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Do 217 K-3:

Similar to K-2, but capable of carrying both Henschel Hs 293 or Fritz X.
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Do 217L:

Modified version of the Do 217K with rearranged cockpit and defensive armament. Only two built.

Do 217M:

Bomber; Do 217K with 1,750 PS 1,287 kilowatts (1,726 hp) DB 603A inline piston engines.
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Do 217 M-1:

Equivalent to the K-1.
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Do 217 M-3:

DB 603A-engined equivalent to the K-3.
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Do 217 M-5:

Kehl radio gear-equipped Henschel Hs 293 carrier with a single missile mounted semi-externally beneath the fuselage.
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Do 217 M-11:

Similar to the K-2 with extended wingspan, Kehl control gear and Fritz X missiles.
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Do 217N:

Night fighter based on the Do 217M. Armament similar to Do 217J but with improved 20 mm MG 151/20 cannons replacing the 20 mm MG FFs plus the addition of four 20 mm MG 151/20s as Schräge Musik.
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Do 217 N-1:

Similar to J-2.
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Do 217 N-2:

N-1 with defensive gun turret and bomb bay equipment removed to reduce weight.
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Do 217P:

High-altitude reconnaissance aircraft with two 1,860 PS 1,368 kilowatts (1,835 hp) DB 603B supercharged by a D B 605T in the fuselage (the Hohen-Zentrale Anlage, or "HZ" system) with a ceiling of 16,155 metres (53,002 ft). Three Do 217 P-0 pre-production aircraft only.

Do 217R:

Redesignated Dornier Do 317 prototypes used as Henschel Hs 293 carriers.

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