This work examines the evolution of flight and takeoff in birds and pterosaurs from the perspective of resistance to mechanical loading in the proximal limb bones of both the forelimb and hindlimb. A comparative approach is taken, using the average section modulus of the humerus and femur of each species as the primary comparative variable. Particular emphasis is given to the problem of giant size in pterosaurs:​ the azhdarchid pterosaurs of the Late Cretaceous included the largest flying animals known from the history of life, and their ability to fly and launch from level ground has been previously contentious. Fluid theory and physiological estimates of fuel usage are also used to generate estimates of maximum range in pterosaurs. Among living birds, penguins have the strongest proximal limb elements relative to mass, but this may be related in part to adaptations for ballast. Most avian clades are indistinguishable according to their limb bone strengths, but major ecotypes among birds can be distinguished in some cases according to the section moduli of their long bones. I find that prior models equating the aerodynamics of pterosaurs and birds are likely flawed. Bird humeral strength scales at a lower rate with increasing body size than femoral strength, while pterosaurs show the reverse trend. Combined with a review of the prior literature on pterosaur gait, I use these results to conclude that pterosaurs were most likely quadrupedal launchers, unlike living birds. This explains giant size of the largest pterosaurs when compared with the largest flying birds.