A novel boundary layer flashback model is developed based on previous measurements that showed flashback limits may be related to strained premixed flame extinction. According to the model, flashback occurs at the equivalence ratio where the strained extinction limit flame speed matches the mean axial flow velocity one thermal distance from the wall. The model is validated by comparison with experimental measurements of flashback of confined nonswirling turbulent hydrogen-air flames. This comparison shows that the proposed model is capable of predicting confined turbulent boundary layer flashback across a large range of wall velocity gradients and preheat temperatures. The model is extended to methane-hydrogen-air flames in a swirling configuration using information about a single flashback event and shows good agreement with experimental measurements as a function of both hydrogen mole fraction in the fuel and pressure. In addition, inclusion of a mean nonreacting velocity field computed via large Eddy simulation allows for a significant increase in the accuracy of the model when applied to swirling flows. Ultimately, this model provides a new pathway for the design of flashback resistant gas turbines, even with the addition of fuels like hydrogen.