One of the more successful field modifications performed on the B-25Cs and Ds were conversions to heavily-armed strafers.
The basic concept for the strafer seems to have originated with B-25 units based in Australia. Medium-altitude bombing attacks against Japanese shipping had not been all that successful, since most of the bombs tended to miss their targets. This was due partly to the fact that medium and high-altitude bombing was subject to inherent errors in accuracy due to uncertain winds and to difficulties in sighting, but also due to the fact that ships could often see the bombs coming their way and had enough time to get out of their path. General Kenney felt that the development of skip-bombing techniques would give a much better chance of success. In skip bombing, the pilot approaches the target ship at a speed of 200 mph and at an altitude no higher than 250 feet off the water. Releasing the bomb at that height or lower caused it to skip off the water and slam into the ship just above the waterline, giving a much better chance of a hit than conventional bombing from medium altitudes. However, this technique required a low-level straight-on approach against intense antiarcraft fire from heavily-armed ships. It was felt that heavy forward-aimed firepower aboard the attacking aircraft was needed to counter this defensive fire.
This technique had already been tried out to a limited extent with the A-20 Havoc, but the A-20 had a relatively low bombload and a limited range. In addition, there was a severe shortage of A-20s in Australia and in the entire South Pacific due to the priority of Lend-Lease deliveries to the Soviet Union. The idea of modifying the B-25 as a "strafer" seems to have originated with NAA field service representative Jack Fox and Major Paul I. "Pappy" Gunn of the 3rd Bombardment Group. Fox and Gunn satisfied General Kenney that this was an idea worth trying, and the General gave them authorization to proceed.
B-25C serial number 41-12437 was chosen for the initial tests. Since in a low-level, high-speed attack the bombs would be released by the pilot, there was no need for a bombardier. Consequently, the bombardier position was removed and replaced with a package of four fixed 0.50-inch machine guns with 500 rpg and aimed directly forward. The guns protruded from a metal plate that replaced the flat bomb-aiming panel. In addition, four more fixed 0.50-inch machine guns were installed in individual external blisters, two on each side of the fuselage. Blast protection from the fuselage blister guns was achieved by using blast tubes on the gun barrels and by mounting large sheet metal plates on the fuselage sides that covered the entire blast area. The plane was appropriately named "Pappy's Folly". In the first tests, the fuselage guns were found to be too far forward for the center of gravity, and were later moved further aft.
Trials were sufficiently impressive for General Kenney to order more strafer conversions. By the end of February 1943, twelve strafers were completed by the Eagle Farms operation in Australia and assigned to the 90th Squadron.
The strafer concept proved particularly effective during the Battle of the Bismarck Sea of early March 1943. USAAF A-20s, B-17s, B-25s along with Australian Beauforts and Beaufighters took part in coordinated and repeated attacks on a Japanese convoy headed from Rabaul to reinforce their forces based at Lae, with P-38s and P-40s flying top cover. The strafer B-25s proved especially effective during this episode, attacking the convoy from nearly masthead altitude using skip-bombing techniques to attack the ships broadside, the withering fire from the eight forward-firing 0.50-inch machine guns preventing any effective return fire. Out of the original convoy of eight destroyers and eight cargo vessels that had departed Rabaul, all the transports and four of the destroyers were sunk or beached. The B-25C/D strafers achieved a 43 percent hit ratio.
Against land targets, these B-25s were rigged with bomb bay cages that contained up to 100 23-pound parachute fragmentation bombs. These bombs were released in great numbers to attack airfield dispersals and flak batteries.
The strafer concept was so successful that by September 1943, 175 B-25Cs and Ds had been converted for low-level strafing by the depot at Townsville, Australia. By that time, five squadrons had been so equipped.
Other commands soon picked up the concept. The 241st Bombardment Group based in the CBI Theatre modified a number of B-25C/D aircraft as strafers with various different nose gun arrangements. They were used with success against railways, marshaling years, highway transport and storage depots. the 41st Bombardment Group of the 7th Air Force in the central Pacific used strafers that were quite similar to those from Townsville.
The concept even reached the Mediterranean theatre of operations, where 16 B-25s were modified by the 26th Air Depot Group in Egypt with a six-gun nose. However, these planes were later returned to standard transparent-nose configuration, which indicates that the "strafer" concept was not all that widely used in the Mediterranean and European theatres.
Following the development of the B-25C/D strafers mounting eight forward-firing guns, Major Paul Gunn developed an experimental installation of three additional guns to the underside of the fuselage between the bomb bay and the forward access hatch. This idea proved to be impractical due to feed belt problems and blast effects to the adjacent structure.
The North American factory came up with the idea of installing a fuselage-mounted module containing two 0.50-inch machine with 225 rpg. The unit fit into the forward access hatch. This installation had the advantage in that servicing of guns and replenishment of ammunition could be done from inside the aircraft, but it had the disadvantage in forcing the crew to enter the aircraft from the aft hatch, then crawl over the bomb bay to get to the forward cockpit. The unit was never ordered into production.
The success of the "strafer" modifications to the B-25C/D led to the B-25G, which was a dedicated factory-built strafer that was succeeded by the more efficient B-25H. However, it was not until the advent of the solid-nosed B-25J that the power of the famous "Townsville" strafers was equalled.