I think bombers are about right as far a toughness go's. If the fighter pilot knows how to attack the bomber it will go down easy enough but they have to take the time to do it right. Most people just want to crawl up the rear of my bombers and I can start hitting them from 1k out.
In short if your having trouble with the durability of your bombers it sounds more like you need to work on your gunnery, start pinging the fighters and they tend to screw up the attack run on you more.