The diminutive XH-20 "Little Henry" was developed by a team led by Marvin Marks in close co-operation between the Helicopter and Propulsion Division of McDonnell and the AAF/USAF Rotor Wing Branch, Propeller Laboratory and Rotary Wing Unit, Aircraft Projects Section, Wright-Patterson AFB. At the time of its inception, the XH-20 was unique in being powered by two McDonnell-developed 18.42cm ramjet units weighing only 4.5kg each, mounted at the tips of the two-blade rotor and fed from tanks beside the pilot. The fuel, originally propane but later gasoline (motor car petrol), was boosted through a feed line to a delivery valve on the rotor head. From there centrifugal force took over to convey the fuel through the blades to the tip-mounted ramjets.
Fitted as a single-seater, the first of two prototypes (46-689 and 46-690) made both its first tethered flight on 5 May, 1947, and its first free flight on 29 August in St Louis with Charles R. Wood Jr. at the controls.
The McDonnell XHJD-1 "Whirlaway," flew on April 27, 1946, with test pilot Charles R. Wood at the controls.