It wasn't a field mod. My Dad's compartment was down near the front of the bombay, he said it was a tough crawl to get there. He had to roll down the radar dome which replaced the belly turret by hand crank, and it took some time, and was done after they were at altitude because of the drag. He said the dome was very heavy, and his guess is when the plane hit the water the dome tore the fuselage in half and he floated out. He was largely unconscious through the night and the flight lieutenant kept him and the copilot afloat until they were rescued by a US destroyer. He really admired the Flight Lieutenant who he mentioned often, an Isadore Lamica from NY state. He was later killed in the war.
The radar was a separate compartment and slaved to the bomb "sight". It sounds to me like a pretty significant mod, probably done at Langley. My Dad's first missions were against German U Boats using B-18s equipped with radar. He flew B-25s a few times there in Virginia, and he commented about how they were like fighters compared with heavy bombers.
I met the copilot once when I was a kid, his forehead sloped back very oddly because he went through the wind shield and apparently Lamica who was not badly hurt followed him on impact. Those three were the only survivors in the crew. They were in the ocean all night in effect, my Dad drifting in and out of consciousness. The official records claim the plane went down shortly after takeoff, but my Dad contends they were aloft because he had rolled the dome down into position and that would have been at least 30 minutes out.
My Dad had been a radioman and had been selected for OCS when he was diverted to something they wouldn't tell him about, radar school. The crew was supposed to shoot him if they were about to be captured by the Japanese, though my Dad said he really didn't know much of anything that would benefit the Japanese.