  We study a new class of rank two sub-Riemannian manifolds encompassing
Riemannian manifolds, CR manifolds with vanishing Webster-Tanaka torsion,
orthonormal bundles over Riemannian manifolds, and graded nilpotent Lie groups
of step two. These manifolds admit a canonical horizontal connection and a
canonical sub-Laplacian. We construct on these manifolds an analogue of the
Riemannian Ricci tensor and prove Bochner type formulas for the sub-Laplacian.
As a consequence, it is possible to formulate on these spaces a sub-Riemannian
analogue of the so-called curvature dimension inequality. Sub-Riemannian
manifolds for which this inequality is satisfied are shown to share many
properties in common with Riemannian manifolds whose Ricci curvature is bounded
from below
