Harley-Davidson first started with William S. Harley in 1901, a 21-year-old who drew up plans for a small 116 cc engine to be used with a regular bicycle frame. Over the following two years, Harley and his friend Arthur Davidson – together with his brother Walter Davidson – built their first motor-powered bicycle, but when it was completed realised that the engine wasn’t powerful enough and required pedal assistance. 
They went back to the drawing board and built a bigger 405 cc engine with a loop-frame design. The first prototype was assembled in the Davidson’s’ backyard and was completed by September 1904 when it took part in a motorcycle race at Milwaukee where it came fourth, signifying the first appearance of a Harley-Davidson on record.   
At the start of the following year advertisements were placed in an automobile trade journal which offered just Harley-Davidson engines, and by April complete motorcycles were being produced on a limited basis. The first Harley-Davidson dealer was from Chicago and sold three of the original bikes built in the backyard shed.  
Their first real factory was built in 1906 on Chestnut Street (Juneau Avenue) and, more than a hundred years later, is still the company’s headquarters. Over the next few years, Harley-Davidson saw a marked increase in sales, as well as production models as they built several prototypes with an 880 cc model selling by 1909 and their production increasing to 1,149 bikes.  
By the time the United States entered World War I in 1917, the military had started making use of motorcycles in combat service, and Harley-Davidson provided the military with over 20,000 bikes during the war. The 1920’s saw them becoming the largest manufacturer of motorcycles in the world, with Harley-Davidsons being sold in 67 countries.  
The start of the Great Depression severely affected their business, with sales dropping from 21,000 to lower than 4,000 in 1933. The company entered into other areas of production in order to stay afloat, such as industrial power plants based on their engine designs and a three-wheeled delivery vehicle named the Servi-Car.  
With the outbreak of World War II, Harley-Davidson, as one of only two US cycle producers to survive the great depression, once again provided the US Army with motorcycles, suspending civilian production during that time. At the time they were supplying the US Army with WLAs, a special military version of their 45” WL line of motorcycles. A trademark of these WLAs is that they all had 1942 serial numbers, regardless of which year they were manufactured in.  
Over 90,000 motorcycles were produced for the military, many of which were provided to Canada and other allied powers including 30,000 of which went to the Soviet Union. The WLAs ceased production at the end of WWII, but was resumed again from 1949 to 1952 during the Korean War.  
Following the war, Harley-Davidson acquired the design of a German motorcycle which they adapted, producing various models which were best known as “Hummers” and were very popular, remaining in production from 1947 to 1966.