A relativistic constituent quark model, formulated on the light-front, is used to derive a new parton approximation for the inclusive semileptonic decay width of the B-meson. A simple connection between the decay rate of a free heavy-quark and the one of a heavy-quark bound in a meson or in a baryon is established. The main features of the new approach are the treatment of the b-quark as an on-mass-shell particle and the inclusion of the effects arising from the b-quark transverse motion in the B-meson. In a way conceptually similar to the deep-inelastic scattering case, the B-meson inclusive width is expressed as the integral of the free b-quark partial width multiplied by a bound-state factor related to the b-quark distribution function in the B-meson. The non-perturbative meson structure is described through various quark-model wave functions, constructed via the Hamiltonian a To appear in Physical Review D.light-front formalism using as input both relativized and non-relativistic potential models. A link between spectroscopic quark models and the B-meson decay physics is obtained in this way. Our predictions for the B → X c ℓν ℓ and B → X u ℓν ℓ decays are used to extract the CKM parameters |V cb | and |V ub | from available inclusive data. After averaging over the various quark models adopted and including leading-order perturbative QCD corrections, we obtain |V cb | = (43.0 ± 0.7 exp ± 1.8 th ) · 10 −3 and |V ub | = (3.83 ± 0.48 exp ± 0.14 th ) · 10 −3 , implying |V ub /V cb | = 0.089±0.011 exp ±0.005 th , in nice agreement with existing predictions.

We derive a new parton formula for the inclusive B decays and briefly discuss its applications to semileptonic and weak radiative decays of the B-meson.