That jet bomber sure is scary, you never usually see the AI do anything with planes! I guess the February patch has taken hold even on older games now, but yeah you have the right idea staying out of sight. Make sure you're a couple of tiles away from their territory and slam them with your own bombers and artillery as you're doing. Using spies is good, or alternatively if you have a high mobility tank you can use it to zoom in and out of their territory to give you vision before retreating to the safety of the fog of war. Just be careful that they can do the same so anti-air is very useful! Battleships also have anti-air capabilities, so if you have any of those then park them next to units you want to protect because land units are considerably weaker on water. Might also be a good idea to get an AT crew because they have some scary looking modern armours so anti-cavalry units may be useful, but go with anti-air as a priority I think. Just make sure you don't make anything else that requires oil until you can get your hands on some Scottish oil though. Once you get drones, they have a sight range of 5 by the way so your artillery and planes can hammer cities from far away even without a tank or spies giving you vision.