The operational Dornier night fighter, redesignated J-1, before entering operations was powered by BMW 801L engines. It was fitted with a revised crew compartment housing a crew of three, with a solid nose housing four fixed 7.92 mm MG 17 machine guns, with four 20 mm MG FF/M cannon in the forward part of the ventral gondola. It retained the MG 131s in a dorsal turret and ventral position of the bomber, and could carry eight 50 kilograms (110 lb) bombs in the rear bomb-bay, with a fuel tank in the forward bomb-bay.

Production had commenced in March 1942, during which eight J-1s were built. In April, 13 followed and 55 were built in May. Despite this start production declined in June and this trend continued until November 1942, when only four were built. Dornier had been ordered to withdraw Dornier airframes for unspecified reasons. Owing to this, by 31 December 1942, only 130 J-1s had been completed. Dornier kept a production run of 19 aircraft for evaluating equipment. These were to be used when Josef Kammhuber, General of the Night fighters demanded the J-1 to have a modified fuselages made available for upward firing cannon installed within the dorsal areas of the fuselage, above the wing roots. This armament configuration was called Schräge Musik ("slanted" or "oblique" Music). A prototype was given four MG 151s in place of its MG FFs and named Do 217J-1/U1. The prototype was modified in September 1942 and sent to the Tarnwitz Experimental Establishment on 14 October for tests on gunnery performance. The guns delivered 125,000 rounds during tests without problems. The concept was available for adoption, although Dornier had some reservations about the slow firing pattern of the MG 151/20.

The Dornier appeared to be a very effective night fighter with significant hitting power. However it attracted strong criticism from the Luftwaffe. The first J-1 was delivered to 4./Nachtjagdgeschwader 1 in March 1942. The crew complained it was too heavy, criticised its take off and landing characteristics. The pilot complained it had "too little performance reserve". The aircraft's high service loading and its poor manoeuvrability in aerial combat did not enhance its performance reputation. Part of the types performance issues lay with the fact the MG 131 defensive guns and bomb release mechanisms had remained, and been built into the J-1 to allow for its use as a bomber. With eight machine guns mounted in the fuselage and the supporting ammunition, the weight was increased and outweighed the Do 217E by 750 kg.

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