As we have seen, in practice Founding-era militias were far from universal, but in the Founders’ conceptual framework the militia consisted of the mass of ordinary citizens, trained to arms and available to serve at the call of the state. As George Mason put it: “Who are the militia? They consist now of the whole people, except [for] a few public officers. ... When the Second Congress sought to exercise its constitutional authority to “provide for organizing, arming and disciplining the Militia,” it directed “each and every free able-bodied white male citizen of the respective states [except for persons exempted under state law and certain other exempted classes] ... who is ... of the age of eighteen years, and under the age of forty-five years” to enroll in the militia of their states. Or as Patrick Henry declared at the Virginia ratifying convention: “The great object is, that every man be armed.”
The Founders’ did have a concept that approximates today’s National Guard — but it was a concept they disapproved. This is the “select militia” — a specially trained part of the citizenry. To the Founders, a select militia was little better than an army. The Philadelphia Convention explicitly rejected a proposal to create a “select militia” for the federal government, as did the Third Congress. The Constitution’s proponents, moreover, repeatedly denied Anti-Federalist charges that Congress’ power to “provide for training” the militia would lead to creation of a select militia.
(legal historian David Yassky; from links provided).