More Info: Because bats have wings and the ability to fly, they are often mistakenly regarded as a species of bird. Though bats do share a few similar characteristics with the bird family, they possess all of the characteristics necessary to be considered a mammal.
What Characterizes a Mammal?
There are an estimated 4500 different species of mammals on Earth all of them sharing the same unique characteristics that classify them as mammals. To be classified as a mammal, an animal must possess two distinct characteristics. They must possess hair or fur, and the mother mammal must have the ability to produce milk to feed her offspring. Most mammals share other common characteristics such as bearing live offspring, they are warm blooded, have differentiated teeth, and have four limbs. 
Because bats share all of these common characteristics, they are classified as mammals. With almost 1,000 species of bats on Earth, they comprise almost 25% of the mammal kingdom. 
How Are Bats Unique Mammals?
Bats are the only mammal that can fly. Unlike birds’ wings, a bat’s wings are more like arms with membranes stretched out to their fingers, which allow flight. Like humans, bats have four fingers and a thumb on each hand, which are as long as the bat’s entire body. Similar to the human anatomy of the finger where flaps of skin at the base of the fingers join them together, bats have a more expansive membrane of skin between the fingers called the patagium joining their fingers thus creating a winged effect.