We propose a method for a QCD based calculation of one-particle inclusive
decays of the form B \to \bar D X or B \to \bar D^* X. It is based on
the heavy mass limit and a short distance expansion
of the amplitudes, which yield a power series in the
parameter 1/M^2_X for the spectra and in
\Lambda_QCD m_b/(m_b - m_c)^2 for the
rates. We study the leading term of this expansion for the
case of the semi–leptonic decays B \to \bar D X l^+ \nu.