  We evaluate analytically the master integrals for double real radiation
emission in the b --> u W* decay, where b and u are a massive and massless
quark, respectively, while W* is an off-shell charged weak boson. Since the W
boson can subsequently decay in a lepton anti-neutrino pair, the results of the
present paper constitute a further step toward a fully analytic computation of
differential distributions for the semileptonic decay of a b quark at NNLO in
QCD. The latter partonic process plays a crucial role in the study of inclusive
semileptonic charmless decays of B mesons. Our results are expressed in terms
of multiple polylogarithms of maximum weight four.
