Fleet assignment and aircraft routing are two sequential steps in airline capacity planning. The fleet assignment model allots the scheduled flights covered by a type of aircraft on the basis of aircraft availability, and the aircraft routing model generates a route for each particular aircraft to ensure that the path-specific requirements for maintenance and connection times are satisfied. Although it is known that the sequential method is not able to minimize the overall cost, no results have been reported on the integration of the two steps. Here these two steps are completed simultaneously. A path-based integrated model is presented and tested on real data. A heuristic is proposed to solve the formulation. The numerical test indicates that a significant cost saving can be achieved and that the heuristic shows encouraging promise in solving large-scale real-world problems.