Sikorsky Aircraft's prototype X2 helicopter became the world's fastest helicopter when it reached a speed of 225 knots -- 9 more than the previous record set in the 1980s by a Westland Lynx. From the company's press release:
The X2 Technology demonstrator combines an integrated suite of technologies intended to advance the state-of-the-art, counter-rotating coaxial rotor helicopter. It is designed to demonstrate that a helicopter can cruise comfortably at 250 knots while retaining such desirable attributes as excellent low-speed handling, efficient hovering, and a seamless and simple transition to high speed.
Press Release via DVICE | Photo: Sikorsky | Previously: Full-Sized Electric Helicopter