A new wing composite box beam has been designed based on the WRAST model. The newly designed wing has detailed structural properties, and is used to evaluate the effectiveness of wing extensions and winglets to improve tiltrotor whirl flutter stability. The frequencies and damping characteristics of the new wing are analyzed, and they are comparable to the WRAST wing model. Sizing of wing extension and winglet beam structures has also been conducted. The simulation results have confirmed the effectiveness of wing extension and winglet. They can significantly improve the wing beam bending and torsion mode damping, and the current designs of wing, wing extension and winglet structures are suitable to be used in wind tunnel for prove of concept tests. Structural tailoring of wing extension box beam has also been conducted, and both beam bending stiffness tuning and beam bending-torsion coupling through composite tailoring of wing extension box beam have been found to be able to increase the wing beam mode flutter speed by 35-40 knots.