Successful alignment control of the B4 helical nanofilament assembly in a binary mixture system of bent-shaped and rod-shaped liquid crystals is demonstrated. The aligned nanofilament domains appear as extremely smooth and uniform stripes over millimeters. The interferometric second-harmonic generation microscopy technique is developed and applied to these aligned domains. It is found that not only chirality, but also polarity, are preserved in a single domain formed from a single nucleus. Such an easily processed uniform bulk of the spontaneously symmetry-broken material is intriguing in functional materials science.