@Schulz I'm not sure, but if this is about evaluating WWIIv3, I guess it should stay in Maps & Mods, while if it is about strategies and tournaments, I guess it should stay in the Warclub.
On the matter: wrong.
That would be true if the bomber would still raid even if killed, but, since downed bombers don't do damage, it is 3.5*5=17.5.
So, on long term, a bomber, that costed you 12 PUs, will damage for 17.5 PUs before being lost; yet this only assuming the game lasts forever.
However, you pay the 12 PUs now and, if the bomber placement is at range of what you want to raid, will make averagely 2.917 PUs damage the following round, plus a 5/6 chance of keeping the bomber, for doing the same the next round, and so forth.
This implies a cumulative average damage equal to the following:
(Round: cumulative damage)
(the cumulative damage increment keeps decreasing, because the chance of still having the bomber after one raid is 5/6, after two raids is 25/36, etc. (for example, the probability of still having the bomber at the start of round 8, to make the 7th raid, is only about 1/3, without counting the risk of getting downed on that one))
Since you paid 12 PUs on round 1, you will be back from your investiment only on round 8, and even then only having damaged your enemy less than 1 PUs more than you did to yourself, and you are probably still worse off, for the fact that all your cost was on round 1.
An additional, and very important, matter is the fact that a same amount of PUs is valuable the most the earliest. For example, having 12 additional PUs on round 1 is usually much better than having 12 additional PUs on round 4, since those 12 PUs spent on round 1 allow you to buy units that may give you income and otherwise strategic advantages in the course of all these rounds. I've little doubt that if you could borrow PUs, you would easily accept pretty high interest rates (Civil War gives an example of this).
Regarding the 12.6161 damage done from round 2 until round 8, the average round in which the damage is done would be 4.2902, that rougly means doing those average total 12.6161 damages over those 2 to 8 rounds would be about the same as doing them all on round 4 only (assuming they don't get wasted for being too much, of course). This would mean that in a game ending (for example, by surrender) on round 8, that would be like you spending 12 PUs on round 1 to remove 13 PUs on round 4 from your opponent; hardly a good deal. The game lasting more than 8 rounds would do little to improve the deal, as that bomber being still in existence therafter is increasingly unlikely, and averagely adds less than 1 PUs every additional round, and averagely less than 0.5 PUs per round from round 12 onwards.
So, all the above shows that bombers are a bad investment for bombing raiding only, but this is tempered by the fact that they may act at least as force in being, having mainly also the ability of regularly attacking with 1 dice at strength 4 (even if you never do it, this should oblige your enemy to invest more for defending against your possible attacks, in land or sea).
Talking about WWIIv3 1941, the only power that may make mainly bomber strategy is Americans, and this is due to the fact that alternative strategies for this power have their impact relatively delayed, for a series of reasons. And, even in this case, buing only or almost only bombers with Americans is not much better than shipping land forces, as those bombers will be able to scarcely impact for the regular warfare. However, bombing strategy with Americans is indeed effective, especially if the Germans player buy a lot of armours on round 1 and 2, but this is not due to the bombers being overpowered for their raiding ability, but because Americans have some limits in otherwise using their potential to high effect.
On the other hand, it is a fact that you almost never want to buy bombers with Germans to bombing raid Russians or British (when you purchase such units, it is usually almost only for their movement and attack power).
Of course, if I made some mistakes, anyone let me know.