Though there was (and still is) much debate over the exact circumstances of the origin of the character "Uncle Sam" as used to represent the United States, in 1961 Congress passed a resolution which recognized Samuel Wilson as the inspiration for the symbol.
Samuel Wilson lived from 1766 to 1854, and during the War of 1812 was in the meat-packing business. He provided large amounts of meat to the U.S. Army in barrels that he stamped with the initials "U.S." Alledgedly, someone suggested that the initials stood for "Uncle Sam" Wilson, which led to the idea that "Uncle Sam" symbolized the federal government.
However it happened, Samuel Wilson did not look like the modern image of "Uncle Sam." Thomas Nast, a well-known political cartoonist in the 1800s, produced some of the earliest cartoons of Uncle Sam, but in 1916 James Montgomery Flagg painted the famous "I WANT YOU" Army recruiting poster for use in World War I. It was later adapted for use in World War II, and that image has since become the standard for how "Uncle Sam" usually appears in most illustrations. Flagg actually used himself as the model for the painting.