Muhammad,

According to John Conway and Richard Guy's "Book of Numbers" the "British" system is the original, going back to Chuquet in 1484. Around 1650 somebody else - also in France apparently - came up with the "American" system. The French still use the original system, so the Americans didn't adopt their system from them in a fit of revolutionary fervor!
In both systems when you hear "hundred" you don't know just from that where you are. On the other hand when you hear "million" this tells you exactly how far you are through the recitation of the number, as it will appear at most once in the name of any number. In the "American" system "thousand" is a fixed word, in the European system it isn't; this makes the "American" system easier to use, as you have to "keep less in your head". I suspect that this led to that system catching on in North America where the traditions were perhaps less firmly established.

-RD

Muhammad,

There is also a historical note on Wikipedia.

Harley