Kaman's intermeshing rotor helicopters were engineered in great part by a new immigrant: Anton Flettner (he was hired by Kaman as a designer).
Here is his previous design - the Fl-282, which was flown in 1941 and produced in small numbers.
A single-seat version entered service in 1942 - here doing trials from the light cruiser Köln:
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A 3-seat version for the Army was produced from 1944:
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Examples were also flown with a nosecone, a partial plexiglass enclosure, and a fully enclosed cockpit:
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