A first principles theory of electronic transport in quantum well infrared photodetectors (QWIPs) is presented. The model is based on the cascade scattering rate equations approach and uses the translation symmetry to reduce the computational cost of the problem. The model delivers a non-equilibrium electron distribution in both dark conditions and under illumination. The model was tested by simulating a non-conventional broadband InGaAs/AlGaAs QWIP and good agreement with experimental data available in the literature was achieved. The model can be applied to any QWIP structure and can be used for extending the range of materials employed.