We study Type I string theory compactified on a T^6/Z_3 orientifold. The low-energy dynamics is most conveniently analyzed in terms of D3-branes. We show that a sector of the theory, which corresponds to placing an odd number of D3-branes at orientifold fixed points, can give rise to an SU(5) gauge theory with three generations of chiral matter fields. The resulting model is not fully realistic, but the relative ease with which an adequate gauge group and matter content can be obtained is promising. The model is also of interest from the point of view of supersymmetry breaking. We show that, for fixed values of the closed string modes, the model breaks supersymmetry due to a conflict between a non-perturbatively generated superpotential and an anomalous U(1) D-term potential.

Latex, 16 pages, 1 ps figure; references added and corrected, added minor comments; version to appear in Nucl.Phys.B