We evaluate numerically several superconducting correlation functions in a generalized t-J model derived for hole-doped CuO 2 planes. The model includes a three-site term t similar to that obtained in the large-U limit of the Hubbard model but of opposite sign for realistic O-O hopping, For parameters close to realistic values, we obtain strong evidence of superconductivity of predominantly d x 2 - y 2 character. The ground state has a large overlap with a very simple resonating-valence-bond (RVB) wave function with off-diagonal long-range order. This function reproduces the main features of the magnetic and superconducting correlation functions.