To improve the control precision and the dynamic property of bank-to-turn(BTT) missile autopilot in the presence of mismatched uncertainties, a novel peaking-free disturbance observer-based sliding mode controller (PDOSMC) is proposed, which not only is available when the $H_2$ assumption of mismatched uncertainties is invalid, but also avoids the undesirable peaking phenomenon caused by the high observer gain. The simulation results demonstrate that, in the case of high gains, the PDOSMC scheme not only can achieve the same control precision as the DOSMC scheme, but also avoids the control input saturation and poor dynamic property caused by undesirable peaking phenomenon within a DOSMC scheme case.