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Abstract We revisit QCD factorization of B → ππ form factors at large dipion masses, by deriving new constraints based on the analyticity properties of these objects. We then propose a parametrization of the form factors, inspired by the leading-twist QCD factorization formula, that incorporates all known analytic properties. This parameterization is used to interpolate between the QCDF results and the constraints from the B * pole. Based on this interpolation, we predict the B → ππℓν decay rate in a larger phase space region than previous studies could. We obtain a partially-integrated branching ratio up to ℬ ≃ O 10 − 6 $$ \mathrm{\mathcal{B}}\simeq \mathcal{O}\left({10}^{-6}\right) $$ , which implies that a measurement of the non-resonant semileptonic decay is potentially within reach of the Belle II experiment.