According to Wikipedia

There has been considerable controversy over the origin of the Brahma. It appears to have developed in the United States from birds imported from the Chinese port of Shanghai, which were thus known as "Shanghai" birds. It is likely that limited cross-breeding with Chittagong chickens from what is now called Bangladesh gave the Brahma the distinctive characteristics of head shape and the pea comb that distinguish it from the Cochin, another breed that derives from the "Shanghai" birds.

Brahmas were first exported to England in December 1852, when George Burnham sent nine "Gray Shanghaes" to Queen Victoria as a gift.[3] The Dark Brahma variety was developed by English breeders from this stock, and later re-exported to the United States.

The Light and Dark Brahma were included in the first edition of the American Poultry Association's Standard of Perfection in 1874;[2] the Buff variant was added in 1924 or 1929.

The Brahma was the principal meat breed in the US from the 1850s until about 1930. Some birds were very large: weights of about 8 kg (18 lb) for cocks and 6 kg (13 lb) for hens were recorded.