American mastiff was developed by crossing the English Mastiff with a dog that was said to be an Anatolian Mastiff or Anatolian Shepherd.  The resulting puppies had a firm tight lower lip line and did not drool; selective breeding thereafter kept the dry mouth.
The American Mastiff is dry mouthed.  The drymouthness is due to the outcross of Anatolian which occurred early in the development of the breed.  It is a large, massive, and powerful dog. The head is wide, heavy and rectangular in shape. The eyes are amber in color. The ears are rounded and set high on head. The muzzle is medium size and well proportioned to head. The nose is black.  It has a scissor bite. The neck is powerful, and slightly arched. The chest is deep, broad and well-rounded, descending to the level of elbows. The ribs are well-sprung and extend well back. The back is straight, muscular and powerful, with well muscled and slightly arched loins. The forelegs are strong, straight and set well apart. The hind legs are wide and parallel. The feet are round and compact, with arched toes and black nails. The tail is long, reaching the hocks. This dog is strong and driving, yet very agile. Puppies are all born dark, and lighten as they grow older, some becoming very light fawn by age one year; some retain dark hairs.  It is possible to have a small white patch on the foot, chest, or chin.