The Do 217 K-1 was a production night bomber version with the 1560 PS BMW 801L engine. It carried the same crew of four in the re-designed "stepless cockpit" crew compartment, but had a revised defensive armament of a twin-barreled 7.92 mm MG 81Z machine gun in the nose, two single MG 81s or twin-barrelled MG 81Z in beam positions, a MG 131 in the B stand position dorsal turret, the DL 131/1C, and another in a ventral position inserted in a Walzenlafette WL 131/1 cylindrical carriage at the rear of its Bola gondola. 220 were built. It had an average flying weight of 12,700 kg, this aircraft achieved speeds of 520 kph at 5,200 m.

The K-1 was equipped with GM 1 nitrous oxide boost, which increased the K-1s maximum speed by 84 kph at 8,000m at a rate of 100 g/sec. With 50 g/sec the aircraft's operational ceiling could be extended from 8,400 to 9,800 metres. But failure rates of the GM 1 were very high and attention was shifting to other Do 217 variants. So the use of GM 1 soon stopped. Serious shortages of the BMW 801 led to the cancellation of the K series. However, tests with BMW 801ML Motoranlage unitized-mount format radial engines added a supplementary command unit to the usual 801 Kommandogerät engine control unit for each radial, which could engage the booster switch, ignition timing and the weak-rich mixture control automatically, which made the 801L powerplants easy to operate. Oil pressures also triggered the VDM propeller. The blades themselves were 3.9 m. A 3.8 wooden propeller could be used as an alternative although with a slight loss of speed.

The standard 2,165-litre fuel tank could be supported by two fuselage tanks with 700-litre capacity or external tanks with 300 to 900 litres. An all-up-weight of 15 to 16.5 tonnes could be expected. The K-1 would need some 850 to 1,110 m to get airborne. Taking off from a grass strip, an altitude of 20 m was reached after 1,500 m and from a concrete runway it was 1,300. With two auxiliary tanks its speed dropped by 4.5 percent, and with bombs, by 6 percent. Flame-damper equipment also caused serious speed reductions; 7 percent in level flight and 9 percent when at operational ceiling.

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