Staff Writer (Updated: 6/3/2016):
The Bell Airacuda began as a pre-war design attempting to fulfill a United States Army Air Corps (USAAC) requirement for a new, twin-engine, multi-purpose platform. As such, the type was expected to fulfill a variety of battlefield roles including that of bomber destroyer, bomber escort, and ground attack. Designs such as this already existed in some air services of the world - the Germans developed their Messerschmitt Bf 110 for this same purpose.
The new design would require inherent long-range capabilities to take the fight to the enemy and structural tolerances to withstand quick, fighter-like movements and diving. Two engines offered the necessary performance power over a single installation and increased crew and aircraft survivability over enemy terrain. The workload would be spread about two or more crew members. Bell engineers understood the design was to fulfill performance specifications that included a 300 mile per hour maximum speed, a 30,000 foot service ceiling, and a 3,000 inherent operational range. Climb to 15,000 feet was to take 10 minutes or less.
Work on the Airacuda product began in 1936 and the bomber destroyer / bomber escort essentially evolved to what was termed a "heavy fighter" in World War 2 aircraft terms. As the USAAC was soon to adopt the Boeing B-17 "Flying Fortress" heavy bomber (1938), it sought to acquire an aircraft suitable for defending large fleets of these new heavies over enemy airspace by escorting them. While the bomber carried defensive armament all its own, the weaponry truly only allowed a limited defensive net in which to harass incoming interceptors launched by the enemy. An accompanying escort fighter could break from the bomber formation and directly tangle with these harassers while the bombers continued to their target. For the bomber destroyer role, heavy cannon armament would be brought to bear to bring these structurally large targets down with a few well-placed hits to their vital engine components or critical frame areas. Additionally, the practice of dropping bombs over enemy bomber formations was taken into consideration for the new aircraft.
Engineers elected for a traditional twin-engine arrangement in which the engine pair was held outboard of the fuselage in individual nacelles built into the wing mainplane elements. However, the design decision was made to utilize the engines in a "pusher" arrangement in which the engines drove their respective propellers aft of the wing trailing edges - "pushing" the aircraft through the skies. This was the opposite of the more traditional approach which saw front-mounted engine driving a forward-set propeller unit in a "puller" arrangement - "pulling" the aircraft through the skies. The fuselage made up the center portion of the aircraft and contained the primary crew areas, bomb bay, and various systems and subsystems. A single vertical tail fin was affixed at the empennage . The front section of the fuselage housed a glazed-over nose for the cockpit crew. The total crew complement numbered five and would consist of the pilot, a co-pilot (doubling as the navigator), a radio operator (doubling as a fuselage/waist/beam machine gunner, and two dedicated gunners.
The mention of the two dedicated gunners were of particular note for their positions were not in the fuselage proper. Instead, their workspace was arranged at the frontal section of each engine nacelle - the engine residing aft of them. Each man managed a single 37mm M4 cannon installation fitted to the front of each nacelle and to this was added coaxial 0.30 M1919 Browning machine guns firing tracer rounds for aiming. The M4 cannons were installed in hydraulically-assisted mounts for some trainable fire and fed by way of five-round clips. For vision, the lead sections of each engine nacelle was glazed over. 119 x M4 cannon rounds were carried per cannon. Access to the fuselage was by way of a tunnel running from fuselage to engine nacelle.
To drive the aircraft, the initial powerplant of choice became an Allison V-1710-9 outputting at 1,090 horsepower. Each was turbosupercharged and drove three-bladed propeller units through a 5-foot, 3-inch extension shaft. As stated, the engine installations took up the aft portion of each nacelle assembly.