Construction of the first experimental prototype "Natter, Versuchsmuster 1", was completed on 4 October 1944. V1 was subsequently referred to as "Baumuster1" ("BM1") and later still the "B" was dropped and the machine became known as the M1. Most subsequent prototypes were known by 'M' codes, as the later prototypes of the Heinkel He 162 were. Manned glider flights began on 3 November 1944. The first glider M1 was towed to around 3,000 m by a Heinkel He 111 bomber with a cable ("Tragschlepp" mode) at Neuburg an der Donau. The pilot was Erich Klöckner, who made all four documented "Tragschlepp" ("towed") flights.