A self-consistent model of the runaway electron acceleration process that includes electron drift orbits has been developed. Two new effects obtained by the analysis are a shrinkage of the beam radius with time and a limitation on the maximum attainable γ, γmax ≲ (Wce/Wpe)2, in such a beam. For the ORMAK runaway regime data, these results are minor corrections to a previously developed model which did not include drift orbits. However, for higher-energy-density runaway beams, the drift orbit effects may become more significant.