This monograph is a testament to the efforts of many people overcoming multiple technical challenges encountered while developing the XV-15 tilt rotor research aircraft. It is a comprehensive and detailed documentation of more than 40 years of effort at the NASA Ames Research Center designing this unique class of aircraft. The tilt rotor aircraft combines the advantages of vertical takeoff and landing capabilities, inherent to the helicopter, with the forward speed and range of a fixed wing turboprop airplane. NASA sees the tilt rotor as a new type of vehicle that will provide flexibility for high-speed, long-range flight, coupled with runway-independent operations. It takes the reader through the entire history, culminating with the first production of the V-22 Osprey, built for the U.S. Marine Corps, and the BA609 by Bell-Augusta. This publication takes the reader through the early ideas of Leonardo da Vinci through the search for an aircraft with Vertical Takeoff and Landing (VTOL) capabilities. There is abundant historical data about numerous innovative flying machines devised during the 1920s and 1930s, and German projects by Focke-Achgelis and Focke-Wulf. The publication includes illustrations from the 1930 flying machine patent of G. Lehberger, as well as the convertible aircraft patent of Haviland H. Platt (1955). The publication also describes how the XV-3 tilt rotor emerged from the Army/Air Force convertiplane program of the 1950s, and the evolution of tilt rotor, tilt wing, lift-fan, and direct lift versions and the evolution of the XV-3 program, which began in 1951. Flight tests are described in great detail, explaining the problems of stability. The development of tilt rotor aircraft technology involved some of the same factors that led to other important aeronautical accomplishments of this century. The vision of a few individuals in search of a practical and efficient new aircraft design, commitment to their goals, and their willingness to continue to pursue their objective while encountering major technical problems and programmatic challenges were critical ingredients in this tale. However, the unique aspect of the tilt rotor story was the combined Government and industry focused effort that was sustained for over four decades to explore, comprehend, develop, and refine this technology. The remarkable product of the investment of public and private funds, and the efforts of the people dedicated to the concept, is an aircraft type that will have an impact on civil and military aviation that will rival the introduction of the practical helicopter more than 60 years ago.