The Mil Mi-12 was the largest helicopter ever build.
(Actually the Helicopter would 've been called Mil Mi-12 if it went into series production. As there were only two prototypes build, the correct name for them is Mil V-12 [or Mil W-12]).
The two prototypes were build in 1967 and they held several world records (for example being the helicopter that carried the heaviest payload)

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