So as you may have assumed, bats required a lot of special adaptations to reach their current state-- echolocation, low-density bones like birds, giant ears, etc etc. But scientists recently discovered that they also (like flying birds, but unlike flightless birds) have genes that improve their metabolic efficiency-- 23% of their genes for breaking down nutrients have been modified since their last common ancestor with other mammals. This was an important step in the process of going from energy-free gliding to highly energy-intensive flapping and sustained flight. Since we don't see bats in the fossil record until their current form, it's hard to tell which adaptations came first, but probably having the energy to control their flight was pretty essential early on.
Darwin apparently spent a lot of time considering the evolution of bats, so it's a good thing that people are working on it now from a genetic perspective.
Full article: http://www.wired.com/wiredscience/2010/04/bat-flight-evolution/