  We study a configuration of D-branes in string theory that is described at
low energies by a four-dimensional field theory with a dynamically broken
chiral symmetry. In a certain region of the parameter space of the brane
configuration the low-energy theory is a non-local generalization of the
Nambu-Jona-Lasinio (NJL) model. This vector model is exactly solvable at large
N_c and dynamically breaks chiral symmetry at arbitrarily weak 't Hooft
coupling. At strong coupling the dynamics is determined by the low-energy
theory on D-branes living in the near-horizon geometry of other branes. In a
different region of parameter space the brane construction gives rise to large
N_c QCD. Thus the D-brane system interpolates between NJL and QCD.
