Operationally, the first fixed forward gun was installed on a B-26 of the 22nd BG in May of 1942. A single .50 cal, mounted on the floor in the nose compartment of B-26 MA #40-1421, "YEAH!". Interestingly, it was not copied by other 22nd BG aircraft.
The 69th Bomb Squadron based in New Caledonia, modified all their B-26B MAs with this installation in late 1942.
In September 1942 an AAF directive ordered all B-26B, B-26B-1 and B-26C to be fitted with a single fixed forward .50 cal, and four removable fuselage mounted .50 caliber "package" guns.
The 70th BS, in Fiji, retrofitted their B-26B MA with the package guns in October-November 1942, but did not install the fixed nose gun. They found that the recoil was too much for the skin of the aircraft, and reduced the armament to two package guns by January 1943.
In Alaska, after losing two aircraft to AA attacking shipping around Kiska in October 1942, at least one 73rd BS was modified with two .50s and two 20mm mounted in the nose. (Some sources mention several so modified, but I have only seen pictures of one.)
B-26B, B-26B-2, B-26B-3 and B-26B-4 assigned to the 17th, 319th 320th and 322nd BGs were retrofitted with the single fixed nose gun prior to deployment overseas, though most lacked the four package guns.
B-26C and B-26B-10 and later blocks were fitted with the fixed nose gun and the package guns as standard equipment.
There were some experimental fittings of other armament, including a pannier with four 20mm guns fitted under the bomb bay, but these never got past the test stages.
