A soybean variety designated 91B42, the plants and seeds of soybean variety 91B42, methods for producing a soybean plant produced by crossing the variety 91B42 with itself or with another soybean plant, and hybrid soybean seeds and plants produced by crossing the variety 91B42 with another soybean variety or plant, and the creation of variants by mutagenesis or transformation of variety 91B42. This invention also relates to methods for producing other soybean varieties or breeding varieties derived from soybean variety 91B42 and to soybean varieties or breeding varieties produced by those methods.