The only mammal naturally capable of true and sustained flight is:

Such mammals as flying squirrels, gliding possums and colugos can't actually fly: they just glide for short distances. Only Bats are capable of true and sustained flight. They belong to the order Chiroptera, the second largest order of mammals on Earth. Their forelimbs help them to fly, as they form webbed wings. But, unlike birds, bats don't use their entire forelimbs to fly: they just flap their spread-out digits, which are very long and covered with a thin membrane or patagium. Bats can fly at speeds of up to 60 miles per hour!

There are approximately 1,240 bat species divided into two suborders: the less specialized fruit bats, or flying foxes, and the highly specialized and echolocating microbats. About 70% of bats feed mostly from insects. The vampire bats feed on blood (such a practice is called hematophagy) and some species, for example fish-eating bats, prefer feeding from animals. Listening to echoes, bats can find their food in complete darkness. More than half of the bat species living in the USA are considered endangered.