The mushroom cloud will probably be from the Pe-8 Bomber which carries a monster bomb called the FAB-5000, nicknamed the mini nuke in game. You can fly the aircraft online sometimes, and it's great for completely screwing up the enemies airfield, unfortunately you need to be quite high (at least 1000m, preferably more) to avoid being blown up yourself. plus whenever the enemy sees a Pe-8 they know whats coming and throw everything at it, and it will break up fairly easily, especially against FW190s and BF109s. B17s are much stronger usually.