  We report on the progress of our calculation of form factors for the
exclusive semileptonic decay of $B$ mesons to pions on $2+1$ flavour lattices
with spacings from $0.080~\mathrm{fm}$ down to $0.044~\mathrm{fm}$. Using the
M\"{o}bius domain wall fermion action for all quarks, we simulate pions with
masses down to $230~\mathrm{MeV}$ and extrapolate to the physical bottom quark
mass by utilizing a range of heavy quark masses up to $2.44$ times the mass of
the charm quark. We discuss the dependence on the pion mass, heavy quark mass
and lattice spacing in our form factors results.
