The very first B compiler was written in BCPL by Ken Thompson. B later became self-hosting, i.e. the BCPL compiler compiled the B compiler, but this had another set of challenges due to the extreme memory constraints. It was an iterative process where a new feature was added such that it pushed the memory limit and then the compiler was rewritten to use the new feature to bring the memory usage down.

C was heavily inspired by B and I suspect written in B aswell. Alternatively, BCPL was extremely portable as it compiled to OCode (what we'd recognise today as bytecode) so that might have been another option. The assignment operators of =+ are straight from B and later changed to += due to Dennis Ritchie's personal taste.