Harley-Davidson models. In the late summer of 1929, Harley-Davidson introduced its 45 cubic inches (737 cm³) flathead V-Twin to compete with the Indian 101 Scout and the Excelsior Super X. This was the "D" model, produced from 1929 to 1931. Riders of Indian motorcycles derisively referred to this model as the "three cylinder Harley" because the generator was upright and parallel to the front cylinder. The Great Depression began a few months after the introduction of their 45 cubic inch (737 cm³) model. Harley-Davidson's sales fell from 21,000 in 1929 to 3,703 in 1933. Despite this, Harley-Davidson unveiled a new lineup
Harley-Davidson ULH, and the 45-cubic-inch (740 cc) R to be renamed W. In 1941, the 74-cubic-inch (1,210 cm³) "Knucklehead" was introduced as the F and the FL. The 80-cubic-inch (1,300 cc) flathead UH and ULH models were discontinued after 1941, while the 74 inch (1210 cm³) U & UL flathead models were produced up to 1948. One of only two American cycle manufacturers to survive the Great Depression, Harley-Davidson again produced large numbers of motorcycles for the US Army in World War II and resumed civilian production afterwards, producing a range of large V-twin motorcycles that were successful both on racetracks