In the 1950s, the US Air Force was interested in developing a VTOL aircraft and awarded a contract to Hiller Aviation, already known for their work with helicopters, to develop an experimental vehicle.
Hiller was tasked with building a VTOL aircraft that could carry a total payload of 8000 lbs, including 35 combat troops or vehicles and equipment of equivalent weight, have an operating radius of 425 miles, a cruise speed of 300 mph, and the ability to hover at 6000 ft. It was also supposed to be controllable in the event one engine failed and be able to make a “controlled crash” landing.
While the initial plan called for a four-engine aircraft, the X-18 was designed to have only two, most likely because it was a test platform for tilt-wing VTOL technology and not a production vehicle. Such a vehicle, the Ling-Temco-Vought XC-142, would later be developed and tested based on the insights gathered from the X-18 program.