There are 2.54 cm in an inch so 16.39 cc in a cubic inch (2.54X2.54X2.54).
520 cubic inches X 16.39 = 8.52 litres or 8522 cc.
All US aircraft engines give their configuration and size in the same way.
B-17 has a R-1820 which is a Radial of 1820 ci
Some vehicles using engines of aircraft origin use the same system such as AVSD-1790, air cooled, V configuration, supercharged, diesel of 1790 ci capacity, or AO-895, air cooled, opposed cylinder, 895 ci.