Pat's question is all you need to know about what kind. Is the stave mostly sapwood, or mostly heartwood?
Either will make a bow, but the "red" elms that have more heartwood are softer, scruffier, and the wood is fibrous, and not as elastic in compression. Elms with mostly white sapwood are harder and have better elasticity. Both have good tensile stength.
Just work toward a nice long, wide flatbow, or mollie, like 2" wide, and comes up to your eye. Or a 2-1/4 or wider R/D flattie or mollie that comes at least to your chin. Longer, thicker bend in the handle designs are good, too. The more heartwood you have, the more you have to fudge upwards on the numbers, and if the wood feels hard, strong, and cuts like frozen butter instead of tearing and shredding, you can fudge down a bit.
