From the DARPA website:
The versatility of helicopters and other vertical take-off and landing (VTOL) aircraft make them ideal for a host of military operations. Currently, only helicopters can maneuver in tight areas, land in unprepared areas, move in all directions, and hover in midair while holding a position. This versatility often VTOL aircraft the right aerial platform for transporting troops, surveillance operations, special operations and search-and-rescue missions.
Compared to fixed-wing aircraft, helicopters are slower-leaving them more vulnerable to damage from enemy weapons. Special operations that rely on lightning-quick strikes and medical units that transport patients to care facilities need enhanced speed to shorten mission times, increase mission range, reduce the number of refueling events and, most important, reduce exposure to the adversary.
By their very design, rotary-wing aircraft that take off and land vertically have a disadvantage achieving speeds comparable to fixed-wing aircraft.,,,”For the past 50 years, we have seen jets go higher and faster while VTOL aircraft speeds have flat-lined and designs have become increasingly complex,” said Ashish Bagai, DARPA program manager. “To overcome this problem, DARPA has launched the VTOL X-Plane program to challenge industry and innovative engineers to concurrently push the envelope in four areas: speed, hover efficiency, cruise efficiency and useful load capacity.” “We have not made this easy,” he continued. “Strapping rockets onto the back of a helicopter is not the type of approach we’re looking for…This time, rather than tweaking past designs, we are looking for true cross-pollinations of designs and technologies from the fixed-wing and rotary-wing worlds.
Excerpt from DARPA EXPERIMENTAL AIRCRAFT PROGRAM TO DEVELOP THE NEXT GENERATION OF VERTICAL FLIGHT, February 25, 2013
See also https://www.fbo.gov/