AMA Class C racing was established in 1933 as a response to declining participation and motorcycle sales during the Great Depression. The class used a displacement equivalency rule that allowed flathead side-valve engines up to 750 cc while restricting the more technically advanced overhead valve designs to 500 cc. This effectively leveled the playing field between American flathead machines and European OHV imports, making racing accessible to amateur riders who could buy the same machines as factory teams.
Harley-Davidson's immediate predecessors to the KR were the WLDR and the race-only WR (1941–1952), which produced around 35 horsepower and formed the direct competitive lineage. Development of the new Model K street motorcycle began around 1950 in response to a growing influx of British brands such as Triumph, Norton, and BSA entering the US market after World War II. The KR racing variant was developed alongside the K street bike, adopting modern features including a hydraulic fork front suspension, rear swingarm, four-speed transmission, and unit construction that European machines had long employed.
The KR used a 45.125 cubic inch (739.47 cc) V-twin engine with side-valve (flathead) architecture, a layout Harley-Davidson favored for its proven reliability over long-distance American racing on rough roads with low-octane fuel. Critics noted that Harley-Davidson remained outside the mainstream of engine development; their first use of overhead valves on a production motorcycle came after OHV engines had already ceased winning AMA TT races, with overhead camshaft designs becoming the only competitive technology in road racing thereafter.
The KRTT was the faired road-racing variant of the KR. A 1963 KRTT road tested by Cycle World achieved a top speed of 142 mph (229 km/h), a 0–60 mph time of 5.8 seconds, and covered the quarter mile in 14.1 seconds at 97 mph.
In 1956, Harley-Davidson KR machines won every single AMA Class C race held that season. Between 1955 and 1969, KRs claimed 12 of the 15 AMA Daytona 200 national championships, establishing the model as the dominant American racing motorcycle of its era. The KR's tenure ended in 1970 when it was replaced by the Harley-Davidson XR-750, which would go on to become one of the longest-running and most successful American racing motorcycles ever built.
The KR's cultural significance extended beyond the racetrack. A 1957 KR was selected for the Guggenheim Museum's landmark 1999 exhibition "The Art of the Motorcycle" in New York City. A 1963 KR featured in the Las Vegas edition of the same traveling exhibition.
The AMA Motorcycle Hall of Fame's Classic Bikes collection preserves two notable examples: a 1959 KR that was the last motorcycle raced by three-time AMA Grand National Champion Joe Leonard, and Mert Lawwill's 1969 KR750, representing the final year of the model's production before the XR-750 transition. These machines serve as physical witnesses to an era when American flathead engineering, operating within rules that favored it, dominated domestic professional motorcycle racing for more than a decade.