(February 17, 2024) The silent B entered English via Latin and Latin got it during its development from the mixing of Druid Akkadian with Indo-European. In writing this is seen during the late Aegean Island writing style as it was being adopted for use in both the Greek and Latin writing. The silent B was used to indicate a diphthong, that is, a transition in vowel sounds within a word.
Some examples of English words with a silent b are "thumb, lamb, coulomb, debt, and subtle." Other silent B words are paired with grammatical forms which sound out the B such as climb-clamber, crumb-crumble, doubt-dubious.
All these words come from Latin. For example doubt and dubious comes from Latin grammatical forms dubiosus, dubius, dubitare, and dubium. Its B became silent when the noun form took on a diphthong sound now actually sounded out in the English word "doubt" by the "ou" spelling instead of the silent b. Significantly, the equivalent French word "doute and douter" left out the B in favor of the /ou/ consturction when the modern forms of spelling developed after the development of printing.
Up to now the origin of the silent b has been uncertain and thus controversial.
https://linguisticsgirl.com/no-doubt-the-silent-b-owes-a-subtle-debt-to-history/
(February 17 2024) Text of right leg of Male Bronze Statue of San Casciano. Letter style is late Aegean Island transitioning into Latin style.
(February 11, 2024) This is mostly late Aegean Island in style just starting its transition to the Greek style. The dual use letter Yod (I,Y) is being split apart into its vowel (I) and consonant (Y) forms. This seems to be the earliest evidence for that.