The Spanish letter B has two pronunciations, depending on where it’s found in the word.
1. When the B is at the beginning of a word or immediately follows a consonant, it is pronounced like an English or French B.
2. When the B is in any other position, it is what linguists call a bilabial fricative.* To make this sound, put your lips close together – not quite touching – and push air through them.
* Bilabial means that the sound is produced with both lips, while fricative means that the sound is created by a partial blockage of the air flow. To give you an idea, the letters F, S, and V are fricative sounds in English.
B is pronounced exactly the same as V in Spanish.
Please note that this explanation is only a guideline for the Spanish that I know, which is Castilian Spanish. There are many regional variations in Spanish pronunciation.