  We present an analytic computation of the two-loop QCD corrections to
$u\bar{d}\to W^+b\bar{b}$ for an on-shell $W$-boson using the leading colour
and massless bottom quark approximations. We perform an integration-by-parts
reduction of the unpolarised squared matrix element using finite field
reconstruction techniques and identify an independent basis of special
functions that allows an analytic subtraction of the infrared and ultraviolet
poles. This basis is valid for all planar topologies for five-particle
scattering with an off-shell leg.
