In 1965, Lockheed answered a US Army Request For Proposals with the CL-945 concept, a stowed-rotor helicopter design. At low speeds and hover, it would operate as a helicopter.
source.image: Found And Explained
At higher speeds, aircraft-style propulsion systems would power it forward, and at much higher speeds than helicopters normally attain.Watch the video from Found And Explained for more info:
The rotors would stop rotating and would fold back and be stowed for minimum drag. It was a great idea that had the little problem of being horribly complex, heavy and expensive.
Wing or Turbo fans for jet propulsion which would have seriously increased its power and just how fast did Lockheed think this would go up to 800 kilometers per hour or just shy of 500 miles per hour that’s tripled the speed of the helicopter that the military was fielding at the time it would have two crew members and a side door that search and rescue teams could repel from to enter the jungle.