When Japan attacked Pearl Harbor and waged war on the USA, a new theatre of war was opened in the Pacific Ocean, with most of the B-18 bombers based overseas in the Philippines and Hawaii. Most of those based in Hawaii were destroyed on the ground during the initial Japanese attack, just like the ones based in the Philippines, and the few ones that remained, played no significant role during subsequent operations.
The B-18 based in continental USA and the Caribbean, were deployed in a defensive role, in order to prevent any possible attack in the American mainland. However, those attacks never took place and the Boeing B-17 replaced the B-18 in the frontline service in 1942. After this, 122 B-18As were modified for anti-submarine warfare operations. The bomb aimer position was replaced by a search radar fitted inside a large radome and magnetic anomaly detector (MAD) equipment was sometimes fit in a tail boom. Those aircrafts, known as the B-18B, were deployed and used in the Caribbean on anti-submarine patrols. On 2nd October 1942, a single B-18A, piloted by Cpt. Howard Burhanna Jr. , belonging to the 99th Bomb Squadron, dropped depth charges and sank the German submarine U-512 north of Cayenne, off the shores of the French Guiana.
In the anti-submarine role, they were replaced, in 1943 by the Consolidated B-24 Liberator, which offered much better payload and had greater range, thanks to which, the mid-Atlantic gap was finally closed.
Surviving B-18s of the USAAF were eventually used as trainers and transports within continental United States. A total of two B-18A were reformed as unarmed cargo transports and received the denomination of C-58. An improved version, named XB-22, powered by the Wright R-2600-3 radial engines was proposed, back in 1938, but it was never built as Douglas focused on the B-23 Dragon light bomber.
After the war, the remaining bombers were sold as surplus on the commercial market and some of them were used as cargo or crop-sprayers by commercial operators.
Sources:
1. https://en.wikipedia.org/wiki/Douglas_B-18_Bolo
2. https://www.valka.cz/Douglas-B-18-t72780
3. http://www.aviation-history.com/douglas/b18.html
