The flashback of premixed flames in ducts is investigated both experimentally and theoretically. New experimental results, compatible with classical results on the subject, are presented. A minimal model for premixed flame propagation in a boundary layer is presented. The scales of the problem are derived from asymptotic considerations. Numerical results for the flame propagation velocity are given; in particular we give the critical condition for flashback. The numerical results are compared to a first experimental visualization of the flow around a tilted flame stabilized in a tube close to the flashback condition. The good qualitative and quantitative agreement between numerical simulations and experiments indicates that the model has captured the essential physical features.