1787: Sept 17, The Constitutional Convention, called to organize a stronger federal government for the United States, approves the Constitution, which is then submitted to the individual states for ratification. Among the powers specifically vested in the new government is "to provide and maintain a Navy" The process of ratification is completed in the summer of 1788, but years will pass before a navy is actually established.
By the way, The US Navy is the only military service that is garanteed by the constitution to exist.