Transition blazars exhibit a shift from one subclass to the next during different flux states. It is therefore crucial to study them to understand the underlying physics of blazars. We probe the origin of the multi-wavelength emission from the transition blazar B2 1308+326 using the ∼14-year long γ-ray light curve from Fermi and the quasi-simultaneous data from Swift. We used the Bayesian block algorithm to identify epochs of flaring and quiescent flux states and modelled the broadband spectral energy distributions (SEDs) for these epochs.  We also investigated its multi-band variability properties and γ-ray flux distribution, and the correlation between optical and γ-ray emissions. We observed a historically bright flare from B2 1308+326 across the optical to γ-ray bands in June and July 2022. The γ-ray flux distribution was found to be log-normal. The optical and γ-ray emissions are well correlated with zero time lag. The synchrotron peak frequency changes from ∼8 × 10^12 Hz (in the quiescent state) to ∼6 × 10^14 Hz (in the flaring state), together with a decrease in the Compton dominance, providing a hint that the source transitions from a low-synchrotron peaked blazar (LSP) to an intermediate-synchrotron peaked blazar (ISP). The SEDs for these two states are well fitted by one-zone leptonic models. The parameters in the model fits are essentially consistent between both SEDs, except for the Doppler-beaming factor, which changes from ∼15.6 to ∼27 during the transition. We conclude that an increase in the Doppler factor might cause both the flare and the transition of B2 1308+326 from an LSP to an ISP blazar.