The largest helicopter ever built was a massive aircraft developed in the Soviet Union during the 1960s called the Mil V-12. The V-12 was a rather unusual test vehicle that featured two rotors mounted side by side at the ends of a large wing. Each rotor had a diameter of nearly 115 ft (35 m). The helicopter was so enormous that the distance from the edge of one rotor disk to the other was almost 220 ft (67 m), even wider across than the wingspan of a Boeing 747!

[VIA]