We report on the model-based development of a climbing robot that is capable of performing dynamic vertical and lateral climbing motions. The robot was designed based on the two-arm vertical-climbing model inspired by the dynamic climbing motion of cockroaches and geckos, with the extension of introducing the arm sprawl motion to initiate the lateral climbing motion. The quantitative formulation of the model was derived based on Lagrangian mechanics, and the numerical analysis of the model was conducted. The robot was then built and controlled based on the analysis results of the model. The robot can perform the behaviors predicted by the model in which the climbing speed decreases when the swing magnitude increases, and the lateral climbing motion can be initiated when the arm sprawl motion is introduced. The experimental validation of the robot confirms that though the reduced-order two-arm model is abstract and ignored various empirical details, the model is sufficient to predict the robot behavior. This conclusion further suggests that the behavior development of the robot can indeed be explored and evaluated by using the simple climbing model in the simulation environment in place of extensive trial-and-error on the physical robot.