A simplified physical model for the steady-state propagation of an infinite fire front through a uniform forest fuel bed in still air is derived from a mechanistic approach that considers a forest fire as a compressible, reactive and radiative flow through a multiphase medium. This model, named the PIF97 model for shortness, includes the effects of the buoyancy induced gas flow on the preheating of the unburned fuel. Fuel is composed of one type of motionless particles uniformly distributed in a fuel bed of constant depth. The conservation equations used in the model are integrated over the fuel bed depth. The spatial domain is divided into the preheating zone ahead of the fire front and the flaming combustion zone. In the preheating zone (model A), pyrolysis and chemical reactions are neglected, and the gas flow is assumed to be one-dimensional. In the flaming combustion zone (model B), some average parameters over this zone are given in order to simplify the description of physical and chemical processes. Models A and B are coupled to form the PIF97 model. The predictions of this model are compared with experimental rates of spread measured during laboratory fire experiments in pine needle fuel beds. That shows the accomplished progress by comparison to the predictions of a purely radiative model and also the limits of the PIF97 model. Under the present experimental conditions, this model correctly predicts the effect of surface-to-volume ratio, and may predict the effect of fuel load, but the quantitative effect of slope is clearly underestimated. Possible reasons for the remaining discrepancies between predictions and experimental results are investigated through an analysis of separate predictions of model A and model B.