The affine formation tracking problem for fixed-wing unmanned aerial vehicles (UAVs) is considered in this paper, where fixed-wing UAVs are modeled as unicycle-type agents with asymmetrical speed constraints. A group of UAVs are required to generate and track a time-varying target formation obtained by affinely transforming a nominal formation. To handle this problem, a distributed control law based on stress matrix is proposed under the leader-follower control scheme. It is proved, theoretically, that followers can converge to the desired positions and achieve affine transformations while tracking diverse trajectories. Furthermore, a saturated control strategy is proposed to meet the speed constraints of fixed-wing UAVs, and numerical simulations are executed to verify the effectiveness of our proposed affine formation tracking control strategy in improving maneuverability.