... The Japanese may at times have tasked Bettys as Patrol Bombers (in a pinch) ... The US bombers were NOT used in this role until their production outpaced the demand for them as strategic bombers over Germany (fairly LATE in the war) ... Even then they were used more as ASW convoy escorts than anything else. Standard Issue heavy bombers were NOT equipped as Patrol planes and their crews were not trained for that mission. The CATALINA was the plane that SERVED in the time of need and accomplished great things.

Sorry... but the B-24 was in use as early as 1941 by Great Britain, and by 1942 by the USAAF and the USN / USMC (as the PB4Y-1) in anti submarine and anti shipping roles... and in substantial numbers. Navy Liberators were also eventually equipped with air-to-surface radar to allow them to maximize their effectiveness. Over 900 (depending on which source you use) PB4Y-1's were produced and used in this role throughout the war. In 1942 the Navy took over all maritime patrol operations using the PB4Y-1, The PBJ-1 (B-25) , as well as the PBY.
These crews were trained exactly for the jobs they did... maritime patrol. They were not "strategic heavy bomber" crews. They did not train that way... they did not fly that way. While they did serve as ASW convoy escorts, they were primarily used for maritime interdiction... seeking out and destroying enemy shipping... in BOTH the Pacific and European theatres. And they did it quite well.
It is a fact that far more Catalina's were produced and used as a Patrol Bomber than both the Liberators and Mitchells combined. But what is not factual at all is that the heavy and medium bombers were not "used in this role until their production outpaced the demand for them as strategic bombers over Germany (fairly LATE in the war)"... on the contrary... they served from the beginning... and served very very well at it.
The primary advantage the Catalina had over the Liberators and Mitchells, was its ability to land on the water. This made it vital in search and rescue operations as well as its anti shipping role. That, and it most likely cost less to produce and operate.