We have developed a statistical mechanical model of the force-extension behavior of alpha-helical polypeptides, by coupling a random-coil polypeptide elastic model of an inhomogeneous partially freely rotating chain, with the latest version of the helix-coil transition model AGADIR The model is capable of making quantitatively accurate predictions of force-extension behavior of a given polypeptide sequence including its dependence on pH, temperature and ionic strength. This makes the model a valuable tool for single-molecule protein unfolding experimental studies. Our model predicts the highly reversible unraveling of alpha-helical structures at small forces of about 20 pN, in good agreement with recent experimental studies.