An analytical study of the developed steady flow in saturated packed beds is presented with emphasis on the influence of the microstructure on the (ensemble) average momentum equation. The fluctuating interstitial velocity field is modeled by a spatial Ornstein–Uhlenbeck process and the functional form of the inertial (Forchheimer) drag term is correctly deduced. For Couette flow in porous media of constant porosity, the velocity profile is uniform except near the solid boundaries where boundary layers appear with thickness comparable to the bead diameter. It is proved that the random macroscopic variations of the porosity in statistically homogeneous packed beds enhance the average flow rate when the Forchheimer law holds.