This is the text of the second 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." From its advent until somewhere around the mid to late 20th century it was interpreted universally as an individual right. The founding fathers and others of the time wrote extensively on this and it was not controversial for centuries.
Wiki provides numerous examples of early commentary here. [1] I find the most compelling and clear to be that of Judge Thomas M. Cooley, which I'll include at the bottom due to its length. In brief form: he posits that if the law were constrained only to the militia, and not the masses of people that may comprise it, then it would be quite a pointless amendment as the very government it seeks to protect individuals from could undermine it by inaction or neglect in regards to the formation of that militia.
What happened in 2008 was DC vs Heller. [2] After DC banned guns in 1975, a police officer found himself in a situation where he was able to have a gun during his line of duty but was left unarmed in the increasingly dangerous and deteriorating neighborhood that he lived in. He petitioned the NRA for help fighting the law. They refused, so he went to the Cato Institute. They (Heller along with 5 other citizens) filed suit, it made its way to the supreme court, and the supreme court unambiguously affirmed that it's indeed an individual right.
[1] - https://en.wikipedia.org/wiki/Second_Amendment_to_the_United...
[2] - https://en.wikipedia.org/wiki/District_of_Columbia_v._Heller
Full quote of Judge Cooley:
"It might be supposed from the phraseology of this provision that the right to keep and bear arms was only guaranteed to the militia; but this would be an interpretation not warranted by the intent. The militia, as has been elsewhere explained, consists of those persons who, under the law, are liable to the performance of military duty, and are officered and enrolled for service when called upon. But the law may make provision for the enrolment of all who are fit to perform military duty, or of a small number only, or it may wholly omit to make any provision at all; and if the right were limited to those enrolled, the purpose of this guaranty might be defeated altogether by the action or neglect to act of the government it was meant to hold in check. The meaning of the provision undoubtedly is, that the people, from whom the militia must be taken, shall have the right to keep and bear arms; and they need no permission or regulation of law for the purpose. But this enables the government to have a well-regulated militia; for to bear arms implies something more than the mere keeping; it implies the learning to handle and use them in a way that makes those who keep them ready for their efficient use; in other words, it implies the right to meet for voluntary discipline in arms, observing in doing so the laws of public order."