Catalyzes the reduction of 2,5-diketo-D-gluconic acid (25DKG) to 2-keto-L-gulonic acid (2KLG). 25DKGR-B has higher catalytic efficiency than 25DKGR-A. 2-dehydro-D-gluconate + NADP(+) = 2,5-didehydro-D-gluconate + H(+) + NADPH 2-keto-L-gulonic acid is a key intermediate in the production of L-ascorbic acid (vitamin C). Belongs to the aldo/keto reductase family.