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WORLD WAR I
In 1917, the United States entered World War I and the military demanded motorcycles for the war effort. Harleys had already been used by the military in border skirmishes with Pancho Villa but World War I was the first time the motorcycle had been adopted for combat service. Harley-Davidson provided over 20,000 machines to the military forces during World War I.

THE 1920s
By 1920, Harley-Davidson was the largest motorcycle manufacturer in the world. Their motorcycles were sold by dealers in 67 countries. Production was 28,189 machines. In 1921, a Harley-Davidson, ridden by Otto Walker, was the first motorcycle ever to win a race at an average speed of over 100 mph (160 km/h). During the 1920s, several improvements were put in place, such as a new 74 cubic inch (1200cc) V-Twin, introduced in 1922, and the “Teardrop” gas tank in 1925. A front brake was added in 1928.

In the late summer of 1929, Harley-Davidson introduced its 45 cubic inch flathead V-Twin to compete with the Indian 101 Scout and the Excelsior Super X.

The Depression
The Great Depression began a few months after the introduction of their 45 cubic inch model. Harley-Davidson’s sales plummeted from 21,000 in 1929 to less than 4,000 in 1933. In order to survive, the company manufactured industrial powerplants based on their motorcycle engines. They also designed and built a three-wheeled delivery vehicle called the Servi-Car, which remained in production until 1973. In the mid-’30s, Alfred Rich Child opened a production line in Japan with the 74ci VL, which became Rikuo after the parent company severed its business relations with Harley-Davidson.
An 80 cubic inch flathead engine was added to the line in 1935, by which time the single cylinder motorcycles had been discontinued.

By 1937, all the flathead engines were equipped with the dry-sump oil recirculation system that had been introduced with the 61E and 61EL “Knucklehead” OHV models. This caused the 74 cubic inch V and VL models to be renamed U and UL, the 80 cubic inch VH and VLH to be renamed UL and ULH, and the 45 cubic inch RL to be renamed WL. In 1941, the 74 cubic inch “Knucklehead” was introduced as the F and the FL, replacing the 80 cubic inch flathead UH and ULH models.

World War II
Harley copied the BMW R71 to produce its XA model.
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 and for private buyers.

Harley-Davidson, on the eve World War II, was already supplying the Army with a military-specific version of its 45″ WL line, called the WLA. (The A in this case stood for “Army”.) Upon the outbreak of war, the company, along with other manufacturing enterprises, shifted to war work. Over 90,000 military motorcycles, mostly WLAs and WLCs (the Canadian version) would be produced, many to be provided to allies. Harley produced the WLC for the Canadian military.
Shipments to the Soviet Union under the Lend-Lease program numbered at least 30,000. The WLAs produced during all years of war production would, unusually, have 1942 serial numbers. Production of the WLA stopped at the end of the war, though it would resume production from 1949 to 1952 due to the Korean War. The U.S. Army also asked Harley-Davidson to produce a new motorcycle with many of the features of BMW’s side-valve and shaft-driven R71. Harley largely copied the BMW engine and drive train and produced the shaft-driven 750 cc 1942 Harley-Davidson XA. Due to the superior cooling of an opposed twin, Harley’s XA cylinder heads ran 100 °F (55 °C) cooler than its V-twins. The XA never entered full production: the motorcycle by that time had been eclipsed by the Jeep as the Army’s general purpose vehicle, and the WLA—already in production—was sufficient for its limited police, escort, and courier roles. Only 1,000 were made and the XA never went into full production. It remains the only shaft-driven Harley-Davidson ever made.

Article from content at Wikipedia- http://en.wikipedia.org/wiki/Harley_Davidson – Under copyright of WikiMedia.
“Photographs courtesy of the Harley-Davidson Motor Company Archives.”