The Harley-Davidson, Inc. (NYSE: HOG) is an American manufacturer of motorcycles based in Milwaukee, Wisconsin, United States.
1903 The company considers 1903 to be its year of founding, though the Harley-Davidson enterprise could be considered to have started in 1901 when William S. Harley, age 21, drew up plans for a small engine that displaced 7.07 cubic inches (116 cc) and had four-inch flywheels. The engine was designed for use in a regular pedal-bicycle frame.
Over the next two years Harley and his boyhood friend Arthur Davidson labored on their motor-bicycle using the north-side machine shop of their friend Henry Melk. It was finished in 1903 with the help of Arthur's brother, Walter Davidson. Upon completion the boys found their power-cycle unable to conquer Milwaukee's modest hills without pedal assistance. Will Harley and the Davidsons quickly wrote off their first motor-bicycle as a valuable learning experiment.

Work was immediately begun on a new and improved machine. This first "real" Harley-Davidson motorcycle had a bigger engine of 24.74 cubic inches (405 cc) with 9-3/4 inch flywheels weighing 28 pounds. The machine's advanced loop-frame was similar to the 1903 Milwaukee Merkel motorcycle. They also got help with their new engine from outboard motor pioneer Ole Evinrude. Elder brother William A. Davidson also lent a hand.
The prototype of the new improved loop-frame model was assembled in a 10 by 15-foot (3 by 5 meter) shed in the Davidson family backyard. The machine was functional by 8 September 1904 when it was entered in a Milwaukee motorcycle race, the first known appearance of a Harley-Davidson motorcycle.
1905 In January 1905 small advertisements were placed in the "Automobile and Cycle Trade Journal" that offered bare Harley-Davidson engines to the do-it-yourself trade. By April, complete motorcycles were in production on a very limited basis. In 1905 no more than a dozen machines were built in the backyard shed. (Some years later the original shed was taken to the Juneau Avenue factory where it would stand for many decades as a tribute to the Motor Company's humble origins. Unfortunately, the first shed was accidentally destroyed by contractors in the early 1970s during a clean-up of the factory yard.)

 In 1906 Harley and the Davidsons built their first factory on Chestnut Street (later Juneau Avenue). This location remains the Motor Company's corporate headquarters today. The first Juneau Avenue plant was a modest 40 by 60-foot single-story wooden structure. That year around 50 motorcycles were produced.

1907 William S. Harley graduated from the University of Wisconsin-Madison with a degree in mechanical engineering. That year additional factory expansion came with a second floor and later with facings and additions of Milwaukee pale yellow ("cream") brick. With the new facilities production increased to 150 motorcycles in 1907. That September a milestone was reached when the fledgling company was officially incorporated. They also began selling their motorcycles to police departments around this time, a tradition that continues today.

Production in 1905 and 1906 were all single-cylinder models with 26.84 cubic inch (440 cc) engines but as early as February of 1907 a prototype model with a 45-degree V-Twin engine was displayed at the Chicago Automobile Show. Although shown and advertised, very few dual cylinder V-Twin models were built between 1907 and 1910. These first V-Twins displaced 53.68 cubic inches (880 cc) and produced about 7 horsepower (5 kW). This gave about double the hill-climbing power of the first singles. Top speed was about 60 mph (97 km/h). Production jumped from 450 motorcycles in 1908 to 1,149 machines in 1909.

In 1909, Harley-Davidson introduces its first V-twin bike with an 1000cc engine which developed 7 horsepower and three years later the Bar & Shield logo is used for the first time and patented as a trademark. The construction of the main factory building and what was to become the six stories headquarters of Harley-Davidson starts in 1912, facility located on Juneau Avenue in Milwaukee. The same year, the company starts exports to Japan and the US dealer network extends to 200 branches. 1913 sees the founding of the Harley-Davidson racing department and one year later the company releases sidecars. Harley was becoming very successful, its contract with the US military being one of the reasons.

By the time 1917 came, one third of all Harley Davidson production went to the US army and the same year the sale of Harley-Davidson bicycles. After almost 20 years of existence, Harley Davidson becomes the largest motorcycle manufacturer in the world as their models can be purchased in 67 countries worldwide by 1920. Eight later, in 1928, the public sees for the first time two cam engines on the JD series models, which can reach speeds up to 100 mph at the same time providing front wheel brakes for all their bikes. An important moment in Harley history comes in 1929 when the 45 cubic inch V-twin engine (also known as flathead) is made available on the D model. In 1936, Harley-Davidson introduces the EL model which had a 61cc engine and was later nicknamed Knucklehead and used a year later (1937) by Joe Petrali to set a new land speed record of 136.183 mph. He did not use the stock version, he altered it a little bit in order to set the new speed record. The same year, the production of the first WL models begins. In 1941 America joins World War II and Harley stops making civilian bikes and focuses all its efforts on vehicles for the military. During the second World War which ends in 1945 Harley made a lot of new models for the army, but the most successful army model was the WLA, Harley producing almost 90,000 units