Even Wikipedia doesn't know:
"Rudolf Bayer and Ed McCreight invented the B-tree while working at Boeing in 1971, but did not explain what, if anything, the B stands for. Douglas Comer suggests a number of possibilities:
"Balanced," "Broad," or "Bushy" might apply [since all leaves are at the same level]. Others suggest that the "B" stands for Boeing [since the authors worked at Boeing Scientific Research Labs in 1972]. Because of his contributions, however, it seems appropriate to think of B-trees as "Bayer"-trees."