The title is derived from the names of two dog breeds - Jack Russell and Bulldog. Myrl Redding (John Cusack) is said to have the tenacity of a Jack Russell and the strength of a bulldog when angered.
The Wyoming state flag depicts a bison bull; The word "jack" is an archaic term for a flag; An early version of this flag appears in the film.
The prayer read when Myrl's wife is buried comes from the Canticle of the Creatures, written by St. Francis of Assisi.
The Jack Bull is an updated American adaptation of the 1808-1810 German novella "Michael Kohlhaas" by Heinrich von Kleist, which is itself based on the 16th-century story of Hans Kohlhase. The events and characters are all adapted from the novella.