The silent "b" in English appears in certain words where it is not pronounced, typically when it follows "m" or precedes "t" at the end of a word. 
This spelling convention has historical roots in Old English and Latin. 
In earlier forms of English, the "b" was pronounced, but over time, the pronunciation was dropped while the spelling was retained. 
Some words with a silent "b" are derived from Latin, where the "b" was originally pronounced, but became silent as these words evolved in English.

Inglish completely drops the -b.

Modern English:
    comb: From Old English "camb"
    bomb: From Latin "bombus," meaning "booming sound"
    limb: From Old English "lim"
    climb: From Old English "climban"
    thumb: From Old English "þūma"
    *debt: From Latin "debitum"
    doubt: From Latin "dubitare"
    subtle: From Latin "subtilis"

Inglish:
    coam
    bóm
    lim
    clîme
    þom
    *dett
    daot
    sutle

*The word "debt" in English comes from the Old French word "dette," which in turn comes from the Latin word "debitum," 
meaning "something owed." The silent "b" was added in English during the Renaissance to reflect its Latin origin, 
despite not being pronounced.