After building various Autogiros, Anton Flettner turned his attention to helicopters in Germany during World War II. His ultimate creation was the Flettner Fl 282 Kolibri ("Hummingbird"), which was an intermeshing rotor helicopter. Only 24 were built by the end of the war, but it is considered the world’s first production helicopter.

Source:  "Aviation Art by David Gibbings", Mike Hirschberg, AHS Calendar 2013