The Conventional Convention, convened in 1787 in the State House of Philadelphia for the purpose of drafting a constitution that would provide a legal structure for the newly-established United States of America, was attended by 55 delegates represented all of the states except Rhode Island. Seventy-three had been appointed or invited to attend, but 18 declined to appear. Thirty-nine of the delegates signed the "final" document ("final" is in quotation marks because there would be a number of amendments to the Constitution over the ensuing years), with William Jackson, the secretary, signing as well in order to authenticate the document.
George Washington's role at the Constitutional Convention, following which he would be elected President of the United States, was one of reluctant observer of the proceedings. He was elected president of the convention, but kept his participation to a minimum. Washington had famously retired to his estate at Mt. Vernon, Virginia, and had to be convinced to attend the convention. While he limited his role to one of observer, there was no question that his was an exalted presence, as he sat in an elevated seat wearing his military uniform. While he voted on occasional issues, he remained aloof from the proceedings, not wishing to be seen as establishing a government to which he strongly suspected he would be asked or appointed to lead.
With regard to the date at which the United States began, there are several that can be used, beginning with July 4, 1776, when the newly formed Congress adopted the Declaration of Independence; March 4, 1789, the date the Constitution went into effect; or January 8, 1790, the date on which President George Washington gave his, and the country's, first State of the Union address.
The original thirteen states appointed 70 individuals to serve as delegates to the convention. However, only 55 attended and only 39 signed the actual U.S. Constitution. George Washington was elected to serve as the President of the Constitutional Convention.