Official Standard of the Bullmastiff
General Appearance: That of a symmetrical animal, showing great strength, endurance, and alertness; powerfully built but active. The foundation breeding was 60 percent Mastiff and 40 percent Bulldog. The breed was developed in England by gamekeepers for protection against poachers.
Size, Proportion, Substance: Size - Dogs, 25 to 27 inches at the withers, and 110 to 130 pounds weight. Bitches, 24 to 26 inches at the withers, and 100 to 120 pounds weight. Other things being equal, the more substantial dog within these limits is favored. Proportion - The length from tip of breastbone to rear of thigh exceeds the height from withers to ground only slightly, resulting in a nearly square appearance.
Head: Expression - Keen, alert, and intelligent. Eyes - Dark and of medium size. Ears - V- shaped and carried close to the cheeks, set on wide and high, level with occiput and cheeks, giving a square appearance to the skull; darker in color than the body and medium in size. Skull - Large, with a fair amount of wrinkle when alert; broad, with cheeks well developed. Forehead flat. Stop-Moderate. Muzzle - Broad and deep; its length, in comparison with that of the entire head, approximately as 1 is to 3. Lack of foreface with nostrils set on top of muzzle is a reversion to the Bulldog and is very undesirable. A dark muzzle is preferable. Nose - Black, with nostrils large and broad. Flews - Not too pendulous. Bite - Preferably level or slightly undershot. Canine teeth large and set wide apart.
Neck, Topline, Body: Neck - Slightly arched, of moderate length, very muscular, and almost equal in circumference to the skull. Topline - Straight and level between withers and loin. Body - Compact. Chest wide and deep, with ribs well sprung and well set down between the forelegs. Back-Short, giving the impression of a well balanced dog. Loin-Wide, muscular, and slightly arched, with fair depth of flank. Tail - Set on high, strong at the root, and tapering to the hocks. It may be straight or curved, but never carried hound fashion.
Forequarters: Shoulders - muscular but not loaded, and slightly sloping. Forelegs-straight, well boned, and set well apart; elbows turned neither in nor out. Pasterns straight, feet of medium size, with round toes well arched. Pads thick and tough, nails black.
Hindquarters: Broad and muscular, with well developed second thigh denoting power, but not cumbersome. Moderate angulation at hocks. Cowhocks and splay feet are serious faults.
Coat: Short and dense, giving good weather protection.
Color: Red, fawn, or brindle. Except for a very small white spot on the chest, white marking is considered a fault.
Gait: Free, smooth, and powerful. When viewed from the side, reach and drive indicate maximum use of the dog's moderate angulation. Back remains level and firm. Coming and going, the dog moves in a straight line. Feet tend to converge under the body, without crossing over, as speed increases. There is no twisting in or out at the joints.
Temperament: Fearless and confident yet docile. The dog combines the reliability, intelligence, and willingness to please required in a dependable family companion and protector.
Approved February 8, 1992 Effective March 31, 1992
You can find this at akc.org bullmastiff breed standard.