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We did!
The first U.S. U-boat kill of the war was by a B-18 that took out U-654 on 22 August 1942
You might be interested to know that Popular Mechanics had a good article about flying Artillery in their February 1944 issue. The article was called "Flying Big Gun" (page 105) and covered the history of mounting artillery pieces in aircraft. The article does cover the B-18 project and I'll post that part of the article here:I have seen that picture before and have always wondered if they finished the modification. Does anyone know? Does anyone have a picture of it completed?
- from page 108:
Shortly before 1939, first tests were made with an old-style French 75 under the direction of Captain (now Colonel) Horace A. Quinn, who was put in charge of the project as Chief of the Aircraft Armament Development Section of the Technical Division, Ordnance, under Colonel (now Major General) G. M. Barnes. The gun was mounted in the fuselage of a junked B-18 bomber and fired on the ground to test the reaction of the fuselage to the shock of such an explosion. These early tests were successful, considering the crude equipment.
Encouraged, but still cautious, the Ordnance men next obtained flyable models of the B-18 through the co-operation of the Air Forces, and went ahead with the most dangerous part of the experiment - flight-testing and firing the old cannon.
In every case, Captain Quinn himself insisted upon taking the risk of firing the first rounds from these makeshift mounts, and to his courage goes much credit for the final outcome of these dangerous experiments.
Five different mounts for the old 75 were tested, each more effective and lighter than the last. These were developed through the co-operation of the Watervliet and Rock Island Arsenals.
With traditional Ordnance caution, enthusiasm was curbed during further modification until, in 1940, an improved model was demonstrated before the Air Corps Board at Eglin Field, Fla. As a result of this demonstration, and under the direction of Colonel Barnes as Chief of the Ordnance research program, a number of industrial companies were called in to help with development and manufacturing problems.
Their wholehearted co-operation produced a 75-mm. aircraft cannon more powerful than even the old French 75, with a perfected recoil mechanism and mount that met the peculiar requirements of an aircraft installation. To Victor F. Lucht, an Ordnance Department engineer, goes credit for the recoil mechanism and the working out of the details that assured flawless performance of the assembled gun.
After more extensive ground tests, the new 75 was taken into the air under the supervision of Ordnance personnel. Again Captain Quinn came forward to take the risks attending the first firing aloft. The results justified these risks.
Now the problem was to find a suitable to take this great Ordnance achievement into combat. As this cannon was a fixed weapon, the ship to carry it had to have speed and mobility, with the guts to stand not only the installation weight but also the shock of firing. The size of the installation, too, had to be considered, along with the necessary equipment and accessories and a man to load the gun.
Several manufacturers were consulted. It was a lucky break when North American Aviation came forward to offer the Mitchell B-25 bomber as the steed. It had already been battle-proven on every front. If it would do, many months would be saved getting it into combat.
By the happy design coincidence the Mitchell was ideal. Along the left side of the fuselage, under the Pilot's compartment, it had a tunnel used by the bombardier to get into his position. This might be just the spot for placing the gun.
To North American, then, was given the exacting problem of installing the 75 in a combat aircraft for the first time. Company experts received the drawings of the gun and installation in mid-August 1942, and George Bussiere, staff engineer in charge of ordnance, gathered his able assistants about him and dug in for a session of sweat and toil.
What they really had to work with were the M-4 aircraft cannon and the M-6 aircraft-type mount as developed by the U.S. Army Ordnance Department. What they had to achieve was the attaching of these units to the Mitchell medium bomber.
First the group studied the Mitchell's structure to see if it would do "as-is". It would-on paper-for months before added ruggedness had been built in to support the nose wheel. Tests were made, and the consensus was made that "she could take it".
To accommodate the gun muzzle, a mild steel port was fitted to the nose of the ship at the end of the bombardier's tunnel. The nose itself was entirely enclosed in metal, replacing the transparent sections in the standard model. Above the cannon muzzle, mountings for two .50 caliber machine guns were fitted as auxiliary weapons, and the necessary armor was added for safety.
During the next four months, test sections of the altered fuselage were taken under wraps to isolated sections of the California coast for testing on a 200-yard range. Here, in utmost secrecy, test firings were made with loads ranging from half charges to more than 115 percent of normal combat charges. As the thunder of these explosions died away, the sections were started back for a final check at the factory, where the verdict was pronounced with satisfied grins: "She can take it - and plenty more!".
Just three months from the day when Brussiere first opened the blueprints, test pilot Rudd was firing the 75 over the Pacific.