The article describes full-scale backdraft experiments in a shipping container using methane as a fuel. Numerical modelling has followed the experimental setup. The numerical simulations show the initial gravity current, the ignition, the spreading of flame in the enclosure, the external fireball, and the subsequent decay. The Detached Eddy Simulation (DES) approach has been used to model turbulence. In order to describe the combustion process of the mixture from the local ignition to progressive deflagration, three separate combustion models have been implemented for laminar, low- and high-intensity turbulence flow regimes. The calculated ignition time is slightly shorter than the average ignition time observed in the experiments. The fire front progresses through the combustible mixture, generating a cloud of hot gases that are accelerated from the container into the external environment. The velocity increases up to 20 m/s. When the fire front reaches the door, combustion continues outside the enclosure as the fuel has been pushed through the door. The comparison between the calculated time history of relative pressure and the pressure sensor record shows that the numerical simulations slightly overpredict the flame front speed, with a stronger pressure pulse and higher temperatures than the observations.