Periodically the developers of IL-2 have offered sales through their fairly new promo-code system and today marks the start of another sale. This time, IL-2: Battle of Moscow is 40% off.
The code applies to new owners and owners of IL-2: Battle of Stalingrad (and I guess the few folks who might only own Battle of Kuban?). You can knock off the price at the point of purchase through a special code.
Even if you bought Battle of Stalingrad through the Steam store, you can still purchase this on the IL-2 1CGS website and get access to the deal without messing with your install.