It wouldn't just evolve to spit fire if it still burned its throat and shit no? Maybe they could've developed a way to combine two reactive chemicals made by its body into a stream coming out of glands in its mouth.

Snakes spit venom, why couldn't a highly evolved giant reptile spit chemicals?

I dont even know why im arguing this point.

edit-
Quote Originally Posted by stinkyattic
...I can just imagine St. George battling his big ol' dragon, and the thing turns around and RIPS one at him....
LOL
ROFL
That caused the funniest visual image ever.