On July 9, 1790, Congress passed the Residence Act, which approved the creation of a national capital on the Potomac River. The exact location was to be selected by President George Washington, who signed the bill into law on July 16. Formed from land donated by the states of Maryland and Virginia, the initial shape of the federal district was a square measuring 10 miles (16 km) on each side, totaling 100 square miles (260 km2).
From Wikipedia: http://en.wikipedia.org/wiki/Washington,_dc