[sokoloff]

The country was founded after an ~8.5 year long Revolutionary War from a colonial owner with a large, well-funded, well-equipped, occupying army. The founders (aka the treasonous colonists) had a general "the people are individually and collectively supreme to the federal government" attitude (no doubt significantly in reaction to the monarchy that ruled over them immediately prior) and wrote the following text into the founding documents:

2nd amendment:

A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed.

10th amendment:

The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.

The first 10 "amendments", which are roughly/essentially contemporaneous with the original document, are referred to as the "Bill of Rights".

[solidsnack9000]

The right to bear arms is also part of the 1689 English Bill of Rights. It is a really old right, dating back to the 900s in various forms on the British Isles.