What historical documents was the US Constitution modeled after?

It was modeled after historical *examples*. The delgates to the constitutional convention were familiar with the styles of government in ancient Greece and Rome, of the various European states of their own day, especially Britain, most of which did not have written constitutions. They also knew, of course, the different written charters that the thirteen colonies had had before independence and the written constitutions they had developed after independence. They used their knowledge of all of these examples to develop the Constitution.