The Japanese Navy learned how to make curry from the British Navy. After the signing of the 1902 Anglo-Japanese Alliance, interactions between the two navies led the Japanese to discover that that the Royal Navy served curry aboard their ships. Initially, Japanese sailors and officers ate curry with bread, but the sailors had joined up to eat rice, not bread. Even by the early twentieth century, Japanese living in rural areas were not really in the habit of eating bread for meals. Sailors coming from rural areas recognised bread really as a snack, not as a meal.
Also I've read that originally British navy served stew. Problem is that milk will spoil very quickly (guess pasteurization is not invented yet), so that leaves milk out of the question. Instead, because Britain owned India at that time, they introduced curry powder that have longer storage time. The curry they made are also thicker than the original India curry.
