Philadelphia hosted the Constitutional Convention of 1787.
Delegates met at the Pennsylvania State House, now known as Independence Hall, from May 25 through September 17, 1787. Their stated assignment was to revise the Articles of Confederation, but they ultimately produced an entirely new governing document.
George Washington presided over the convention, while James Madison became one of its most influential participants. The delegates debated representation, executive power, taxation, federalism, and the relationship between national and state governments.
Philadelphia had already been central to the independence movement: the Continental Congress met there, the Declaration was adopted there, and the Articles of Confederation were debated there. The Constitution was signed on September 17, 1787, by 39 delegates representing 12 states.