We describe the design of a superferric superconducting dipole magnet for a large twin-beam proton accelerator: the Superconducting Supercollider. The magnet generates an induction field in the range 0.15 to 3T, uniform within 1:104at 1 cm radius, with a bore 1 × 1.4 in. Each magnet is 140 m. long. The programs POISSON, CUARM and MAGNUS were used in the design, the last one for three-dimensional problems, and also for some two-dimensional ones. We present performance and accuracy information for the different computer models, and results of fringe field and effective length calculations with the program MAGNUS.