Bats are unique among mammals in that they are capable of powered flight. While we do not have evidence of flightless bats, we do have fossils of bats with primitive features. The oldest known species* is Onychonycteris finneyi, which Nancy Simmons and colleagues discovered in Wyoming rocks that date to approximately 52.5 million years ago .
If you’ve ever looked at a bat’s wings in detail, you’ll notice that the scaffold for the wing membrane is actually a hand, with very long fingers extending to the edge.
Despite these being fingers, bats only have claws on their thumbs, with the exception of some old world fruit bats which also have claws on their index fingers. Onychonycteris is more similar to earlier, non-flying mammals in that it possessed claws on every one of its fingers, hence it’s name: “clawed” (Onycho) “bat” (nycteris).
Another transitional feature involves the relative sizes of its limb bones. Bats are typically characterized by having particularly long forearms and legs that are short relative to their arms, features thought to be adaptations for flight. Just by comparing the photos above, it should be apparent that Onychonycteris is not proportioned the same as modern bats. Simmons et al.  compared its limb proportions to modern bats and various non-flying, tree-dwelling species, and they discovered that Onychonycteris was proportioned much more like non-bats than bats (see below).
Though Onychonycteris was almost certainly capable of powered flight, it may have been less adept at flying than modern bats and perhaps spent ample time climbing trees as opposed to just taking to the air.
In short, Onychonycteris, the earliest known bat, had some traits that are more similar to non-flying mammals than bats, consistent with the hypothesis that bats evolved from non-flying species.
*Another bat, Icaronycteris, is approximately the same age, but it has more modern features than Onychonycteris.
Questions for Creationists
Why would God create most bats with only a claw on the thumb, but give Onychonycteris claws on every finger? Is it just a coincidence that this fossil was found in rocks that correlate with 52.5 million years ago and it has some features that are more similar to other mammals than bats? If it could fly, shouldn’t it have survived Noah’s flood?