Good question, I’m very glad you asked. There are about ten words in English that end in –mb, but have a silent B. Off the top of my head, I can think of:
As you can see, it’s a fairly common phenomenon, but what’s the story behind it?
As is often the case with silent letters, the story begins with Well, we actually used to pronounce it. Most of these words had quite different origins (tomb from Greek, climb from Old English, bomb from Italian and not entering the language until the 17th century), but were generally pronounced in a straightforward manner, with both M and B pronounced as you’d expect them to be. In the latter years of Middle English though, the language generally became simpler, probably due to the often competing and contrasting influences of French and German, which led to many confusing pronunciations and spellings. With consonant clusters then, often the sound of one of the consonants would be removed to simplify pronunciation, as different sounds clumped together sounded increasingly ugly in terms of the developing sounds of English. This is why we often don’t pronounce the N after an M, and I think this was due to the increasing influence of the less consonant-intensive French.
Pronouncing the B came to sound a little bit clumsy, as it might do to you now. Of course this is from an English perspective, as a very common error for English learners is to pronounce the B. It’s an understandable one too, because there’s no obvious reason why it shouldn’t be pronounced, and often Romance languages have similar words with an audible B, like bomba in Italian, or, ironically enough, bombe in French. Adding to the confusion is the fact that words with silent B’s often have related words which have retained the pronunciation of the B. Climb and clamber, for example, crumb and crumble, or limb and limber (though this particular link’s debatable).
If you think about it, English is full of many more silent letters, which often have become silent for similar reasons. I won’t bore you with the details about all of those now, because what I find interesting is how willing we are to change the pronunciation of words, but not their spelling. I think this is because throughout history, the written word has always seemed like a special, sacred thing. Partly because for a long time it was so rare. Most people didn’t know how to write, and didn’t have access to the necessary materials. Writing was therefore the preserve of those with economic, political, and/or religious power. As it was so exclusive, it surely gained a certain mystique. I think it also seemed magical because it allowed us to make the abstract concrete. Spoken language is great, but for most of our history we were unable to record it, and it was thus ephemeral, existing only for moments in the air. But to write meant to bring your thoughts permanently(ish) into the world. And how could you dare mess with something so amazing!?
Even today, though we English speakers use a wide variety of accents and pronunciations of many words, we’re still quite conservative about how we spell words. You can see that in the number of people who rail against abbreviations. And though I don’t mind things like txtspk too much, I still hate to see a misspelling in a publication. And while we might not think of the written words as something as magical as we used to, we still give it a lot of respect. Reading is still seen as a slightly la-de-da pastime, and we consider a written message more important than a verbal one. You wouldn’t accept a verbal contract to start a job, would you?
So while it might seem dumb that we spell certain words with a B but don’t pronounce it, I think it’s fairly logical, based on the ways we think and feel about language, so it gets a thumbs up from me.