This is discussed at length in "The Legend of the Justified True Belief Analysis" by Julien Dutant (Philosophical Perspectives 26(1)). He writes at one point (bolded mine):
In 1960, Gilbert Ryle still ascribes the infallible mental state view to the tradition in his “Epistemology” entry for Urmson’s Concise Encyclopedia.
years later, in the “Knowledge” entry for Edwards’s Encyclopaedia, Anthony
Quinton (1967) writes that the Justified True Belief analysis was the traditional
one and that it has been refuted by Gettier. What happened? Woozley (1949,
181-184), Malcolm (1952, 179–80) and Ayer (1956, 21) all took the infallible mental state view to have sceptical consequences. That was deemed unacceptable
and prompted Malcolm, Ayer and Chisholm to defend the idea that fallible
justification and truth were sufficient for knowledge. Gettier (1963, 121n) was
perhaps the first to note that a formally similar account appeared in Plato.
Soon some called the Justified True Belief analysis “traditional” and by 1967
the Legend coalesced.
This suggests that Gettier sparked the idea that JTB was classical and Quinton nailed that stipulation. Dutant also mentions that Malcolm, Chisholm, and Ayer, trying to counter scepticism with foundationalism, modified the actual traditional analysis to essentially become JTB, creating the position Gettier attacks with his cases.
Long story short: Even though there are people invoked as ancestors, the position of knowledge as JTB is one developed in the second half of the 20th century. Probably Malcolm, Ayer, and Chisholm (in that chronological order) are the ones to be 'blamed' for JTB, with Gettier and Quinton being the ones responsible for the rise of The Legend, ie.
Edmund Gettier’s landmark paper successfully refuted the traditional analysis of knowledge. (Sosa et al., 2009, 189, cited in Dutant 2015)