From: Near Portland, OR
My father was in a photo unit that got attached to different bomber units. He rode in the nose of some of those B-25s attacking shipping. His plane never attacked anything big enough to warrant a bomb, but he said the .50s were enough to sink most smaller vessels they encountered.
The advantage of a .50 machine gun is the rate of fire. The holes aren't big, but they can turn the waterline of an unarmored vessel into a colander.
Skip bombing was devised by the legendary Pappy Gunn of the 5th AF. He was tasked with finding ways to maximize the use of the minimal air assets the 5th AF had. Te initial tests for skip bombing were done with B-17s, but they were too valuable and when they hit on replacing the bombadier on a B-25C/D with 4x .50s and another 4x on the sides, they had a great flak suppression and small ship killer. (The B-25J could squeeze a bombadier into the nose with 4x .50s too. It was a tight fit.)
Pappy Gunn also came up with a number of other improvised ideas. A stock of small parafrag anti personell bombs were found in Australia and Gunn managed to get them. The A-20 was initially not very useful, but the 5th AF had a number of them. Gunn put extra guns on them, filled half the bomb bay with fuel, and the other half with a home made bomb rack for the para frag bombs and the A-20s became capable of closing down just about any runway in one pass nd with the range to reach most of the Japanese bases in eastern New Guinea. The Japanese would have to bring out bomb disposal teams to deal with the parafrags that didn't explode before they could fill the many holes created by the ones that did.
WitP AE - Test team lead, programmer