Those final <b>'s are either cases where earlier pronunciations simplified with no change in
spelling or where analogy and over-generalization caused people to respell earlier <m> spellings
of word-final [m] by adding an unetymological <b>. In a number of words from both Old English
and French-Latin the <mb> originally spelled [mb]—for example, climb, comb, dumb,
lamb, and womb (all from Old English), and aplomb, bomb, catacomb, jamb, plumb, succumb,
and tomb (all from French and Latin).
Over time the pronunciation simplified from [mb] to [m] in both final position and in the middle of
inflected forms, as in words like bomb, bombing and plumb, plumbed . However, in some
derived forms the syllable boundary caused the [b] to be heard, as in bombardier, plumbago.
The power of analogy and a tendency to over-generalize led people to add <b> to words in which
final [m] was earlier spelled <m>: limb (OE lim), numb (OE niman), thumb (OE thuma), crumb (OE cruma) (AES, 184.108.40.206).