Finding good sources for something like this is fairly hard, but actually /b/ was the very first board 4chan ever had.[1]
Though, actually, the explanation I gave still doesn't add up, because actually, I don't think Futaba Channel ever had an "Anime / Random" board. Whether there was already a similar nomenclature from another contemporary imageboard like world2ch is unclear this far away in time, but either way, even though an /a/ board didn't exist, presumably moot already "reserved" /a/ for a normal anime board, whether it was inspired by another imageboard or what. It's not too clear since there were a lot of potential choices and /b/ is a strange one in the grand scheme of things, and I don't think moot ever really explained it too well.
Though, actually, the explanation I gave still doesn't add up, because actually, I don't think Futaba Channel ever had an "Anime / Random" board. Whether there was already a similar nomenclature from another contemporary imageboard like world2ch is unclear this far away in time, but either way, even though an /a/ board didn't exist, presumably moot already "reserved" /a/ for a normal anime board, whether it was inspired by another imageboard or what. It's not too clear since there were a lot of potential choices and /b/ is a strange one in the grand scheme of things, and I don't think moot ever really explained it too well.
[1]: https://www.dailydot.com/debug/4chan-10-years-christopher-mo...