I asked on Twitter the other day
What is the historical reason for denoting magnetic filed “B”?
Eric Eekhoff sent me an answer and with his permission I’m copying his email below:
I saw your question on your GrokEM twitter account about why magnetic field was denoted as B. I recall that Maxwell just used the letters A through H for vectors in his Treatise on Electricity and Magnetism and some of them stuck and some of them didn’t. A is still used for vector potential, B for magnetic field (or magnetic induction or flux density, depending who you ask), H for magnetic intensity, etc. Maxwell used C and G for other vectors that I don’t recall at the moment. They, for some reason, never stuck.
Hope that helps. Have a good day,