You should tell her how you feel. But if you're to shy practice in the mirror. If she doesn't feel the same way then just change to subject so things don't get to awkward. Try changing the subject to playing a game.
I think you should see if she gives you any signs that she likes you back. If not you shouldn't tell her about your feelings because it could make your friendship with her feel strange for the both of you. Or even ruin it. I know because same thing happened to me a while back :/
I think that in every boy-girl friendship at some time some of the two likes the other. Be sure if that's not a fling because you maybe are single or because you confuse the tenderness and intimacy that friendships offers combined of course with the feeling that the other one understands you and you have a good time with him/her. That are things you are looking for in a relationship but there should also be sexual attraction. Being compatible as friends doesn't make you a fitting couple for sure. Secondly be sure it's not a fling because if she doesn't feel the same way things don't go easily back to the way they were. OR if something happens and it ends badly you not only lose a girlfriend but a friend too. If you feel strongly about the girl and you are willing to take the risk you've got your answer. I suggest you don't hit on her out of the blue and suddenly present yourself as a new person. That will confuse her... Better tell her that you wanna talk and tell her the truth. That over the time you developed feelings for her, you don't know how she feels, you don't wanna lose her as a friend but you thought you should be honest to yourself, your feelings and to her.
Make your move; see what happens. It will only end your friendship (assuming she turns you down) if you let it embarrass you into walking away altogether. But, if you DON'T make some kind of move, you'll just going on being best friends until she gets a boyfriend who is jealous of the friendship.