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We use the new nilpotent formulation of open-closed string field theory to
explore the limit where the number $N$ of identical D-branes of the starting
background is large. By reformulating the theory in terms of the 't Hooft
coupling $\lambda=\kappa N$, where $\kappa$ is the string coupling constant, we
explicitly see that at large $N$ only genus zero vertices with arbitrary number
of boundaries survive. After discussing the homotopy structure of the obtained
large $N$ open-closed theory we discuss the possibility of integrating out the
open string sector with a quantum but planar homotopy transfer. As a result we
end up with a classical closed string field theory described by a weak
$L_\infty$-algebra, containing a tree-level tadpole which, to first order in
$\lambda$, is given by the initial boundary state. We discuss the possibility
of removing the tadpole with a closed string vacuum shift solution, to end up
with a new classical closed string background, where the initial D-branes have
been turned into pure closed-string backreaction.