Transfers two electrons from NADH to two molecules of cytochrome b5 through an enzyme-bound FAD. Electrons from the reduced cytochrome b5 are then transferred to various electron acceptors, thereby participating in methemoglobin reduction in red blood cells and in the desaturation and elongation of fatty acids, cholesterol biosynthesis and cytochrome P-450-mediated drug metabolism other tissues. 2 Fe(III)-[cytochrome b5] + NADH = 2 Fe(II)-[cytochrome b5] + H(+) + NAD(+) Component of a complex composed of cytochrome b5, NADH-cytochrome b5 reductase (CYB5R3) and MTARC2 (By similarity). Interacts with MTLN; the interaction is required to maintain cellular lipid composition and leads to stimulation of mitochondrial respiratory complex I activity (By similarity). Belongs to the flavoprotein pyridine nucleotide cytochrome reductase family.