  We generalize the Poland-Scheraga model to consider DNA denaturation in the
presence of an external stretching force. We demonstrate the existence of a
force-induced DNA denaturation transition and obtain the temperature-force
phase diagram. The transition is determined by the loop exponent $c$ for which
we find the new value $c=4\nu-1/2$ such that the transition is second order
with $c=1.85<2$ in $d=3$. We show that a finite stretching force $F$
destabilizes DNA, corresponding to a lower melting temperature $T(F)$, in
agreement with single-molecule DNA stretching experiments.
