The 1930s Harley-Davidson Motorcycles Channel features some of the earliest models around. Check out vintage Harleys at HowStuffWorks.
1931 Harley-Davidson Model D
The 1931 Harley-Davidson Model D switched from F-head to flathead (side valve) V-twins. First was a new 45ci model to compete with the Indian Scout. The next year the 61ci and 74ci Big Twins were changed to a flathead design. Read about the Model D.