I'm not familiar with the evolutionary history of bats, but I wouldn't be surprised if the intermediate form was something like the flying squirrel or sugar glider. No need to travel by ground, when you can climb and glide.

Some species can interbreed, but the resulting offspring isn't capable of reproducing on its own. (See mules, and tiglons/ligers.) Rather circularly, that's how we define 'species' in the first place. At some point along the line, one subgroup or another becomes genetically incompatible with the rest.