Low Salary Compared to Coworkers is Eating Me Up

I've read a lot of posts where people in similar situations are told, "Never try to leverage a coworkers' salary into a raise." But here's my situation:

I've been with my company for just under 15 years. I started as an intern, and have worked my way up to an associate position, always being told I'm doing a good job. Whenever a project comes up that needs a volunteer, I jump on it if I can. Finding out my coworkers' salaries is not something where I had to snoop, or heard through the grapevine. When I write a contract for a project, I have to take into account who is going to work on that project and what we bill the client for their time. If I know what we bill the client, it's a straight billing multiplier to know what their salary is.

So here's the thing. All the other employees in similar roles to me make between 18-25% more than me. We all have about the same amount of time with the company, and are on a pretty even playing field otherwise. One person who is more-or-less a step below me in education and responsibility basically makes as much as I do.

Last year I worked roughly 50 hours per week, on a 40 hour per week salary. I was told that although they appreciate all my hard work and effort, the team didn't do that great, so my bonus was mediocre, and although I thought I got a decent raise for my efforts, it turns out everyone else got about the same raise.

So now what? It's completely demoralizing to know that I'm working my ass off, but making a significant amount less than the others. I'm in a position right now where they could not let me go. I'm 12 months into an 18 month project that is making the company a ton of money. Do I say something to my supervisor? "Hey you know I appreciate the bonus and raise I got in December, but I was writing the contract for this other project, and I can't help but notice that this guy and this guy are making way more than I am."

Do I threaten to leave? Coincidentally, the project I'm working on may turn into another job opportunity later this year with another company I'm working with. They have expressed interest in bringing me on with them, but don't want to discuss further until the project we're on is nearing completion. Should I use that info to leverage a raise in the meantime?

I hope all this makes sense and doesn't come off as whiny. I'm kind of ranting right now, but I really am sick of putting in a lot of extra time and effort and not getting compensated at least equally.

Edit: Before anyone tells me to let my actions speak for me, and wait for my next bonus/raise review, I've done that. Last year was my best year ever in how much money I made the company, but if I did the math with how many hours I worked, then I'd have to go back 4 years before my hourly rate was less. Bonus/raises are given out every December. They have already been discussed and decided upon by my supervisor and higher-ups. And every year I graciously accept whatever I'm given, even though I'm not always happy with it. Should I have said something at this last meeting? When is the appropriate time to bring up a raise, when they are given out once per year?