Ok, firstly, you aren't just friends, there is more to it than that. Don't cheat on your BF. You will feel soo much better about yourself, and others will respect you much more for it. Now, the playing field is level because you both have SOs. It is now time for the dramatic moment. You tell him how you feel. He tells you how you feel.
Having it continue as you are is impossible and will hurt you both. If you trust him, If you love him, you are gonna throw your cards down on the table, and he will too.
This could result in a very tender, loving, and fulfilling relationship. Or it could result in getting hurt. But regardless you will be able to move on. Let your boyfriend go before becoming his GF. Let a grace period pass. Do not anger your ex, or let him think it is because you met someone else.
Why are you together with your current bf if you do not love him? You should think about your relationship with him, how it's going. Because maybe you are bailing for the wrong reasons and you are just looking for someone else to fill a void in your life. Just think about it for awhile. After that, do what you have to do.
No matter what advice you follow, it will come down to that moment.
Realize that you aren't and have never been just "friends" there has always been something more. Whether it is purely sexual or romantic is what you have to find out. I got together with a girlfriend under the same situation. Just say how you feel, but be careful how you phrase it. If possible make it so that he must say how he feels about you.