A recent study on archosaur (crocodilians, birds, and dinosaurs) anatomy has shown that theropod dinosaurs gradually evolved longer, heavier, and more muscular arms as their anatomy led towards the development of wings for flight. Interestingly, these theropods also developed a more crouched and bird-like posture along with the stockier arm morphology. These findings certainly lend key anatomical knowledge as to the evolution of birds from nonavian theropod dinosaurs such as the maniraptorans, so I suggest that you read this article for more information on this compelling analysis:
