Yes there were, but not anymore! Flying squirrels are the oldest living line of modern squirrels. Proof of their existence dates back to the late Eocene period, between 38 and 55 million years ago. These animals did not fossilize well, so all that has been found of them are their teeth, but much can be determined from these fossils. Flying squirrels most likely emigrated to what is now know as North America via the "Bering Land Bridge" about 25 million years ago

There is a living species called the flying squirrel, but it does not actually fly. The flying squirrel has large flaps of skin that connect from its front legs to its back legs (see the link for pictures) that help it glide in the air for a time, jumping from tree brance to tree brance.