The short answer is that the right to bear arms was frequently discussed in when the Constitution was written, but under different contexts than our own. There are quite a few primary sources on it that I'll discuss here, but before that I want to get across the idea that, when you're interpreting what the Framers were thinking when they wrote the Constitution, it's a good idea to go back to English Common Law for reference. Most everything in the Constitution has precedence in Common Law, it isn't something Americans just gave up after the Revolution.

Going back to 1689, the English Bill of Rights guaranteed the right to bear arms to Protestants "suitable to their Conditions and as allowed by Law." This bill also limited the ability of the monarch to maintain a standing army, and gave Parliament the ability to regulate ownership of arms. 18th century jurist William Blackstone published a commentary on English laws. On the subject of the right to bear arms, he saw this right as part of the "natural right of resistance and self preservation, subject to suitability and allowance by law." In other words, the idea that arms were a method of resistance to tyranny was always combined with the idea of a militia organized for national self-defense, and this was regulated to some degree or another. This is the legal context the Framers were working with.

Now, as far as contemporaneous primary sources go, the one that comes to mind first is Federalist No. 4, authored by James Madison under the pseudonym Publius. In it, Madison says the following:

"To these (a standing army) would be opposed a militia amounting to near half a million of citizens with arms in their hands, officered by men chosen from among themselves, fighting for their common liberties, and united and conducted by governments possessing their affections and confidence. It may well be doubted, whether a militia thus circumstanced could ever be conquered by such a proportion of regular troops. Those who are best acquainted with the last successful resistance of this country against the British arms, will be most inclined to deny the possibility of it. Besides the advantage of being armed, which the Americans possess over the people of almost every other nation, the existence of subordinate governments, to which the people are attached, and by which the militia officers are appointed, forms a barrier against the enterprises of ambition, more insurmountable than any which a simple government of any form can admit of. Notwithstanding the military establishments in the several kingdoms of Europe, which are carried as far as the public resources will bear, the governments are afraid to trust the people with arms. And it is not certain, that with this aid alone they would not be able to shake off their yokes. But were the people to possess the additional advantages of local governments chosen by themselves, who could collect the national will and direct the national force, and of officers appointed out of the militia, by these governments, and attached both to them and to the militia, it may be affirmed with the greatest assurance, that the throne of every tyranny in Europe would be speedily overturned in spite of the legions which surround it."

Okay, so what Madison is saying here is that an armed population is an effective tool for resisting the tyranny of a centralized government. He is also making the argument for state militias to keep the federal army in check, and against having a large, standing army. Alexander Hamilton voiced the same opinion in Federalist No. 28.

Similarly, Noah Webster wrote in another Federalist pamphlet with a title so long I'm not even going to bother typing it here, that:

"Before a standing army can rule, the people must be disarmed; as they are in almost every Kingdom of Europe. The Supreme power in America cannot enforce unjust laws by the sword; because the whole body of the people are armed, and constitute a force superior to any bands of regular troops than can be, on any pretense, raised in the United States."

Patrick Henry, George Mason, and Richard Henry Lee voiced similar concerns about a large standing army and the possibility of an armed citizenry being able to contest it.

Now, in my view, the Framers were looking at things from the perspective of having just won a major rebellion. Part of their problem with the British was their large army, and the Framers felt that such a standing army was generally just dangerous to liberty. If you don't have a large standing army, then what option makes sense for defending the country from outside threats? State militias were a potential solution to that problem. But they also were clearly conceived of as having the function of being able to resist federal tyranny should the need arise.

It's really complicated and up to your interpretation. For the record, I personally don't care much for their logic on this particular point, but they said what they said and I've tried to represent that here in good faith. Anyways, those are a few sources contemporaneous with the drafting of the Constitution that can help.