  We study the semileptonic $b\to u$ transition in the decay mode $B^-\to
\pi^+\pi^-\ell^-\bar\nu_\ell$. We define $B\to \pi \pi$ form factors in the
helicity basis, and study their properties in various kinematic limits,
including form factor relations in the heavy-mass and large-energy limits, the
decomposition into partial waves of the dipion system, and the resonant
contribution of vector and scalar mesons. We show how angular observables in
$B^-\to \pi^+\pi^-\ell^-\bar\nu_\ell$ can be used to measure dipion form
factors or to perform null tests of the Standard Model.
