We have constructed several hopping machines using a new type of resilient, flexible leg that we call the “Bow Leg.” The Bow Leg (patent pending) comprises a curved leaf spring, foot, freely pivoting hip, and the “Bow String” that holds the leg in compression. The leg spring is used for multiple purposes: as the leg structure, as the elastic element to store and release ground collision momentum, and as an energy accumulator to store thrust actuation energy during flight. This design features high energy efficiency and low-power actuation, and it has enabled the development of hopping robots that carry all power on-board. This paper focusses on the design of the one-legged 3D Bow Leg hopping machine currently under development. Specific issues include three-freedom control of the flexible leg using tension elements. The prototype is a work-in-progress that has demonstrated short hopping sequences on level ground under remote control.