The A variant was a pre-production reconnaissance version, with original shallow fuselage and powered by DB 601A engines. Armed with three MG 15 machine guns. Nine built, entering service in late 1940, and used for secret reconnaissance missions over the then neutral Soviet Union. Although the specifications had originally envisaged a multi-role aircraft which could perform bombing missions, it was to function in reconnaissance roles. The fuselage had to be extended to accommodate the two cameras which could be "accessed directly by the crew" (presumably in flight). Dornier was ordered to produce three A-0 series machines up to the E variant. This changed to six as the number of reconnaissance machines was inadequate for the military's need. The small production run would consist of six aircraft. The original power plants, the DB 601Fs, could not be installed in time and the lower performance DB 601B engines were assigned to the type instead for the short term. As in the Dornier Do 17, Rb 20/30 and 50/30 cameras were to be installed. The 20/30 would be fitted in the fuselage while the 50/30 camera was to be placed in the cockpit and be jettisonable. For emergencies the aircraft was to have a fuel transfer unit control installed to move fuel from one tank to another. The first four A-0 aircraft were powered by DB 601Bs while the last two were given DB 601N engines in January 1940. The tests were problem free. However the RLM requested a B variant design which would have a fairing in which the film footage would be stored. Dornier reported that the A-0s were not getting the power plants they required for high altitude reconnaissance sorties so construction had to be delayed. The Do 217B program was suspended, as it turned out, altogether.
Dornier completed work on the V6 prototype, the fourth A-0 aircraft. On 15 October 1939 it was flown successfully. The bomb bay had been enlarged and continued testing various weaponry until 1941. It had DB 601P engines installed and its wing was enlarged in early 1941. The DB 601Ps could operate a maximum altitude of 5,800 m and would use high octane C3 aviation fuel. It should have been ready to fly by March, but problems with the engines slowed progress. At that time Dornier was working on additional features, in particular a new pressurised cabin for the A variant. It also hoped to introduce GM-1 booster units to increase performance at extreme altitudes. The first trial of this aircraft took place on 23 April 1940.
The RLM had stated it wanted the A prototypes to be tested by May 1940 with its high altitude engines. Owing to unresolved engine complications testing was delayed. Finally, on 1 October 1941 RLM ordered Dornier to return the aircraft to its original condition and abandon high altitude modifications. In January 1942 Dornier was ordered to prepare the two Do 217A aircraft fitted with DB 601Fs for immediate combat operations in both transport and bomber roles. The conversion was to be complete by February but the aircraft were not operational. Finally one machine was made ready and began operations with DB 601F engines on 30 December 1942, some ten months later. The other machine was sent to Löwenthal in July 1940 and had undergone trials with autopilot controls. From March 1941 it had been fitted with BMW 801A-1s and later A-2 engines for greater reliability on long range sorties. In the summer of 1942 BMW 801G-1 engines were installed while later it had trials with BMW 801G-2 engines which had GM-1 boosters fitted in January 1943 for high performance at altitude. A number of the A prototypes served as test beds through the war. The last (V7) flew testing improvised de-icing systems at altitudes of 9,000 metres. In December 1944 the Dornier projects were halted owing to lack of fuel.
In 1940–1941 the A-0s that were allocated to combat units saw service mainly in Western Europe serving in Kampfgeschwader 2 from bases in the Netherlands.