Bats are unique mammals which evolved body shape and bone properties for flight. The arms of bats, especially the radius and the metacarpals of fingers, are elongated to form the wing. The material and structural properties that allow flight were analyzed, including cross-sectional shape parameters, bone density, and Young’s modulus We hypothesized that there is a proximo-distal gradient in structural and material properties of the bat wing bones, allowing flexibility at the end of the wings. The bones in the wing were analyzed, including the humerus, radius, metacarpals and first and second phalanges. Microcomputed tomography (micro-CT) was used to determine bone shape;​ quantitative backscattered electron microscopy(qbSEM) was used to determine bone density and nanoindentation was used to determine modulus. We found the density and modulus decreased from proximal side to distal between bones. Density variations of all the bat bone samples matched modulus changes both within bones and between bones. This unique gradient in material properties allows bats the mechanical flexibility required for flight.