I pulled a copy of the JAHS article, too, and it's interesting for its references to the Hughes XH-17 heavy lift helicopter...according to the article, Kellett originated the concept, but it was turned over to Hughes in 1948 for development. The KH-15 was built under a US Navy research contract to pursue Kellett's goal of developing automatic stabilization for helicopters -- a real challenge. The article (dtd Jan 1956) says that the KH-15 had been flown for up to four minutes "hands off."
Note on nomenclature. Stéphane's correct. USAAF helicopters were designated as "R-x" (for Rotary Wing) until 1948, and "H-x" thereafter. In either case, the "X" prefix indicated that the aircraft was a prototype. It's clear from the article that Kellett's internal designation system used "KH-x" (for Kellett Helicopter) throughout.