A new lightning return-stroke model based on a distributed circuit approach is formulated. The DNUTL (Dynamic Non-Uniform Transmission Line) model allows the computation of the time and height dependence of the channel parameters and contemplates the corona sheath and core losses representation. The capacitance and inductance of the channel are calculated by applying an original approach based on simple recursive equations. Electromagnetic fields generated by the DNUTL model at several distances from the channel base are presented for a given set of parameters. The obtained results indicate that the proposed model is able to successfully reproduce the main features of electromagnetic field waveshapes associated to natural and triggered lightning.