A novel soybean variety, designated XB31B11 is provided. Also provided are the seeds of soybean variety XB31B11, cells from soybean variety XB31B11, plants of soybean XB31B11, and plant parts of soybean variety XB31B11. Methods provided include producing a soybean plant by crossing soybean variety XB31B11 with another soybean plant, methods for introgressing a transgenic trait, a mutant trait, and/or a native trait into soybean variety XB31B11, methods for producing other soybean varieties or plant parts derived from soybean variety XB31B11, and methods of characterizing soybean variety XB31B11. Soybean seed, cells, plants, germplasm, breeding lines, varieties, and plant parts produced by these methods and/or derived from soybean variety XB31B11 are further provided.