Staff Writer (Updated: 5/18/2016):
Interestingly enough, the IL-2 began as a two-seat aircraft project, but was then modified to only accommodate one crewman. Apparently the armored 'bath' that the pilot and rear gunner sat in proved to make the aircraft much to heavy in this role - one can imagine that a slow moving low altitude aircraft became a sitting duck to both ground and air fire. Thusly the rear gunner position was removed entirely for the first production models. This single-seat IL-2 model was the IL-2 that became available to Soviet air crews at the start of the German invasion of the Soviet Union in 1942.
With the invasion in full swing, production of the IL-2 hit a peak of 300 aircraft per month. In the first few months of operation, the aircraft was found to be extremely susceptible to rear attack from enemy fighters and the rear gunner position (with applicable 12.7mm machine gun) was reinstituted in the IL-2M. The rear gunner position was added in an extended cockpit area behind the pilot with a fuel tank separating the two positions.
As mentioned, the rear-firing gunner area featured a .50 caliber (12.7mm) positional machine gun with a somewhat limited arc of fire. Aircrews were known to completely remove the rear-ward part of the canopy to allow the rear-gunner and increased field of vision. The implementation of the originally designed rear-ward firing position afforded the IL-2 an increased loitering time over the battlefield as the system could now offer up a defense against oncoming enemy fighter escorts.
Several aerodynamic refinements were instituted in further models and the 20mm wing mounted cannons replaced by 23mm types. Improvements to the bomb-dispensing cell doors in the wing-underside were also implemented, allowing for a quicker response times of the 200 dispensing anti-tank bomblets. Provisions for 4 x 132mm rockets replaced the 8 x 82mm rocket armament arrangement from before. A torpedo bomber and a two-seat trainer version followed.
The IL-2 became an highly respected and highly feared adversary. Soviet air crews dubbed the machine the "Flying Tank" for its incredible ability to withstand a tremendous amount of damage and still release its payload only to return home intact. Naturally, one would think this would be related more to the structural core of the armored bath cockpit but the overall structural design contributed greatly to the systems resilience. Likewise, the German infantry referred to the Sturmovik as the "schwarzer tod", or "Black Death" for its very capable payload arrangement.
As improvements were made to the airframe and tactics, so too did the armament improve. More powerful cannons were integrated as were more potent rockets and bombs. The system proved so effective, in fact, that most of the Soviet aircrews piloting IL-2's were recipients of the "Gold Star of Hero of the Soviet Union" award.
At war's end, it was reported that upwards of 36,000 IL-2's were produced for the Red Air Force (1943-1944 alone reported up to 12,000 IL-2's in operational service). The aircraft more than returned the value of initial investment and went on to become the symbol of the oft-forgotten East Front in World War Two. The much improved IL-10 would go on to supplant the successful IL-2 by 1944, but the IL-2 would still remain in service in one capacity or another through the end of the war.