Wilson Sporting Goods Co., with the help of famed Notre Dame head coach Knute Rockne, developed a new double-lined football in 1924. The following year, the first valve-inflated football eliminated the need for a stem, which had always caused a lump under the laces. With Rockne's name on the ball, the "KR," as it was known, was easier to throw in a tight spiral, and it paved the way for the modern passing game.
In 1935, the NFL shortened the ball's short axis to between 21-1/4 and 21-1/2 in. The ball's length was shortened that year to between 11 and 11-1/4 in., and the amount of air that it could hold was set at 12.5 to 13.5 psi. Its long axis of 28 to 28-1/2 in., however, did not change. These dimensions remain today, as does the ball's shape, which has been called a "prolate spheroid" since 1890.
In 1941, with "Slingin'" Sammy Baugh of the Washington Redskins fast becoming the NFL's first great passing quarterback, Wilson purchased "The Duke" name from rival Spalding. Ever since, Wilson has been the official manufacturer of all NFL footballs. The Duke, also used by the American Football League from 1960 to 1969, received a name change in 1969 when the two leagues merged. It is now simply called the NFL ball. Wilson continued to improve the ball with such innovations as hand-sewn ends, triple lining and lock-stitch seams. In 1951, rubber footballs were tried briefly, and in 1956, the NFL approved the use of easier-to-see white footballs for night games. This evolved into special night footballs with white stripes around each end—banned in 1976 because the paint made the balls slick. In 1955, Wilson developed the TD football, which featured a new Tanned-in-Tack material, also known as Grip-Tite. This material has a tacky feel that makes the ball easier to grip, especially when wet. The exclusive Tanned-in-Tack cowhide leather is supplied by the Horween Leather Co., and has been since 1941. That's right. No pigskin. In 1981, a new Ultra Pebble design for the leather, which further enhanced the grip, was introduced. Inside the ball is a high-tech three-ply polyurethane bladder. Two of the layers are standard polyurethane but the third is a unique material, which is secret. In fact, Wilson's bladder recipe is such a well-kept secret that factory tours in that area of the facility are forbidden. The lacing, once fragile cotton, like shoelaces, is now made of an extruded polyvinyl chloride, which is more durable.