Some Douglas B-18 and B-18As were modified late in their service lives for antisubmarine warfare (ASW). The B-18 was used because it was available and a patrol aircraft was needed immediately to counter the German U-boat threat off the coast of the United States and in the Caribbean early in World War II.
The bombardier's compartment was replaced by a search radar with a large radome. Some aircraft were fitted Magnetic Anomaly Detection (MAD) units housed in tail booms (as shown in the photograph).
