In late 1940, testing under operational conditions began. By March 1941, 37 Do 217s had been built and test flown. Many of the E-1 variants, now being built in increasing numbers, were selected for conversion to the new improved fighter variants; the planned 217H, P and R series. A large number and these 'fighter/bomber' aircraft were put through severe testing runs between July and September 1941. Dornier was able to gain valuable knowledge for the future improvement of the armament and bomb jettisoning systems. Of the first six prototypes, two (the third and sixth) were delivered to operational units. The third, Wrk Nr. 1003 was lost on 22 May 1941 (at Rechlin) and 1006, the sixth prototype, was severely damaged on 11 April 1941 whilst with Kampfgeschwader 40.