The end of the J series did not mean the end of the Dornier night fighter. One of the few German fighter pilots to side with the type against its critics was Hauptmann Rudolf Schoenert of III./Nachtjagdgeschwader 3. Schoenert suggested to his Commanding officer, in July 1942, that trials be made of weapons slanting upwards at an angle of 70° (later known as Schräge Musik) in the fuselage in the hope of increasing the efficiency of his Do 217. This entailed mounting four to six MG 151/20 guns in the centre of the fuselage. At Technisches Amt, two Do 217s, one with four and the other with six MG/151/20 cannon were ready for inspection on 5 August 1942 and testing in September. The idea of the upward firing cannon had originally come from an engineer, Dr. Poppendieck, in June 1942. Nevertheless Schoenert built on it, and with the introduction of the IR spanner and headlight, the bomber could approach from below a British bomber and avoid exposure to its powerful powered turrets g uarding its tail, nose and upper fuselage by attacking from behind or head-on. Unlike the B-17 Flying Fortress, the British bombers did not have a Ball turret, and the new Dornier design attempted to take advantage. It was decided, due to cost, to limit the upward-firing armament to four guns. Other tactical improvements involved fitting a semi-rigid brake parachute on October 1942, allowing the Dornier to adjust to the speed of the bomber before firing on its target. The prototypes J-1/U2 and J-1/U4 were tested under these conditions.

These designs were to be carried forward into the new variant, the Dornier Do 217N. The BMW 801 that powered the Do 217J proved underpowered, so a night-fighter using the more powerful DB 603 A-1 engines was produced, with the first prototype flying on 31 July 1942. While it had much improved performance, it was still unpopular due to its poor agility and climb rate, and was prone to engine problems. Ten pre-production series N variants were designated as test beds. Trials began in the summer of 1942. On 16 August the second prototype Do 217, N V2, entered trials,. The N V1 and N V2 were the main testbeds, and the DB 603 A-1s they were powered by were tested at high altitude. On 11 October 1942 the N V1 crashed after Stalling with its landing gear down and crashing into Müritz Lake, killing the crew. On 21 December 1942, 100-hour engine endurance trials began at Rechlin with the DB engines. The pistons became useless after 91 hours. Testing of DB 603 A-2 inline engines was carried out between 28 April and 8 May 1943, but the programme was beset by continual breakdowns and the project was abandoned. There was no further record of the N variant prototypes after 20 June 1943.

In April 1943, the four MG FF guns had started but were not completed until the late summer. The third prototype, N-1/U was fitted with MG 151/20 and unspecified aerodynamic refinements. The machine was used in high-altitude de-icing tests, and the aircraft was tested with Lichtenstein BCR and Bernhardine radar. In August ten of these aircraft were constructed, and between 27 and 31 August, they were fitted with their Schräge Musik at Tarnewitz and Wismar Testing Facilities. The tenth N variant, designated N-0, underwent radio trials. The machine was tested with the Peil G VI/APZ 6, a later and more sophisticated variant automatic direction-finding equipment. On 2 December further tactical trials were carried out with infrared target-illuminating equipment. These trials were carried out with DB 601 powered J-1s.

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