A semi-empirical model for a steady linear fire front propagating through a statistically homogeneous porous bed, for the basic case of no wind, no slope, and fine fuel particles, is presented. The model structure is modular, based on an open library of submodels for the relevant phenomena. The submodeling of moisture heat absorption and radiation is addressed in detail. Presently, the model is not closed; it requires two properties of the fire front as inputs. Various sensitivity analyses and parametrical studies are performed. A laboratory test in a bed of pine needles showed that the model predictions are very sensitive to the flame height estimation and that the empirical submodels available in the literature are not adequate. However, if the flame height is considered an input, the rate of spread and the profile of the leading edge of the front in the fuel bed are predicted within the experimental errors.