B was based on BCPL, which I assume is where the name came from.
But the actual A language would be APL, which stands for A Programming Language. It's a few years older than B and was pretty popular, so it could've influenced the name.
Gotta admire the wherewithal of continuing down the alphabet rather than just "Another Programming Language", through "Yet Another Programming Language" to "Dude, Where's My Programming Language".