This paper presents research done towards the goal of achieving automatic flight control for commercial airliners in formation flight. The motivation for this research is to ultimately reduce fuel-consumption through a reduction in the drag of the follower aircraft, which is a result of the formation flight. Traditional aerodynamic equations for conventional flight of fixed-wing aircraft are expanded to include formation flight interactions. A trim analysis uncovers risks, challenges and feasible trim regions for the formation follower to maintain. These regions include a potentially risky region which is sandwiched between two untrimmable regions, with respect to a maximum aileron setting, and an outside region which has only one untrimmable bound, making it less risky but with lower fuel-consumption benefit. Next, a state space representation is constructed, allowing for a linear dynamics analysis. The poles and their movement as a function of the lateral and vertical separation of the follower aircraft relative to the leader aircraft are shown, and indicate greater changes in flight dynamics due to vertical separation than to lateral separation. The results of the trim analysis and linear dynamics analysis form the basis for the design of a formation flight control system.