In late 1936, the Do 17 E-1 began to be mass-produced. The first Luftwaffe units to receive the bomber were Kampfgeschwader 153 (KG 153), and KG 155. Aufklärungsgruppe (F)/122, a reconnaissance unit, began converting to the Do 17 F-1. In early 1937, KG 255 was formed, also to be equipped with the Do 17 E-1.According to the Generalquartiermeister der Luftwaffe, 479 Do 17s were on strength. Some 100 more had been lost in crashes or sent to Spain. On 12 March 1938, Do 17Es of KG 155 dropped pro-Nazi leaflets on Vienna, as a prelude to the city's occupation during the German Anschluss operation. KG 153 received its first Do 17 E-1s on 20 September 1938 as part of a mass re-equipment program (note the significance of the date). The unit was redesignated KG 3 on 1 May 1939. KG 155 converted on