Second-harmonic generation (SHG) was observed in an unwound ferroelectric smectic C* liquid crystal using two counter-propagating fundamental waves. A remarkable enhancement occurred when SH light wavelength approached to the optical pitch of the helicoid. This enhanced signal based on a helicoidal distributed feedback action is proportional to the fourth power of the sample thickness in thick cells, and becomes larger compared with the conventional phase-matched SHG signal in relatively thick samples. The crossover points of the SH intensities in the two phase matching types shift toward thinner cell thickness in materials with a larger dielectric anisotropy.