Why is the D silent in words like Bridge?

The letter "J" was created by an Italian in 1524, initially as an alternative to the roman numeral I.  Its usage as a typical alphabet letter in English became common in the 17th century.  Before this addition, to make the sound J in English, we used the letters "dg."  You can typically tell how old a word is by whether it uses the "dg" or J in its spelling; the newer words use the letter J.