In most professional positions, your job description describes, using high level language, what the position most likely entails. However, most of these job descriptions also have one other element to them, which generally states that the position can also include "other duties as assigned".
In short, it's really your duty to do the work that's assigned to you, and most employers want to hire people who they know won't just be fair-weather employees who will be there for the fun stuff but who will hide when it comes time to do heavy lifting.
While R&D may not be what you're interested in, consider that it's an essential part of the success of an organization. While frustrating, you're likely participating in breaking new ground in terms of what your company does. It takes a good, trusted employee to be able to do this, so if your boss has selected you to do this, then look at this as an opportunity for growth.
In conclusion, I'm not sure if you're not excited about the work because you think you'd do a bad job or because you find it boring, but if it's because you don't think you'd do a good job, consider that R&D is hard, and if your boss thinks you can do a good job at this, then believe in yourself and tell yourself that you can too. ;)
If you're not excited about the work, you can still explain to your boss that, if there's someone else equally qualified and interested, then he should give the work to that person. However, I wouldn't suggest saying no. While you can certainly explain to your boss that you're not interested in the work, you would probably do much better for your career if you also added that you'll do the best you can because you want to do a good job and help the company succeed. If you really really loathe the work, then it may be time to move on. Good luck!