Devil, this is the hard part of "balance". Short of an exact match up of speed and payload each side has to have the objectives aligned with their abilities. It is not unreasonable to put the bomb damage and survival expectations different for each side.
Perhaps having an object quota per attack squad set to reflect the relative ability to attack and survive is a solution?
Yet, as hard as it is, you are already ahead of it.
For example. Let's start by balancing just one side. Let's count how many B25s (or whatever) need to achieve the objectives for Allies. You then plug in expected loss ratios, determine how many should survive, and such. Once the "facts" are established, you plug in the amount of the JU88s that will deliver the same amount of ordinance, with the same chance of success, and the same time to target. Might be 45 bomber formations for Allies per quarter, and 54 Axis, it won't matter, it's the tonnage that you count and balance. That's one way to possibly attack this balance issue. There are others, but it really depends on how Brooke defines the objectives, how close, how long to get to each target (3 hours per segment, need to get 3 flights to target per segment) but the format here is we don't count planes, we count ordinance and ability to deliver. We even have to plug in the formula to define speed so that targets are spread out by each sides ability to get their, not both "25 miles apart", make sure that each side gets to their objectives at the same time regardless of speed.
Then, of course, we need to anticipate the ability to defend the targets, so each thing we solve ads another thing to solve
But, the Start Point is Target Cluster. That defines everything. Identify the objective. Adjust the sides to meet the objectives. Adjust the Objectives if it proves to be something unable to achieve. Great events aren't designed, they evolve.
The ability to shoot down bombers has to be equal. If the JU88s are more in number than the B25s, and half the B25s and half the JU88s have to be shot down, there needs to be enough fighters on both sides to shoot down the bombers, self defense must be counted, all bullets, ordinance and speed need to be identified and accounted for. It's all about the spreadsheet that works in the game itself, not the historical balance, those facts and figures fall apart in game.
It can be done, it's a butt ton of work, and not easy but sure as hell worth it! I think the only downside is, like Nef, Brooke has to deal with ME!
As I wrote this, I saw your latest post Devil. The loadout on attack planes can be controlled. You can enable and disable the guns, bombs, rockets per plane. The objective should be simple, take down the "stuff". Achieve the objective or not. Now, you have to decide, should the event be a level bombing/dive bombing JU88 event and escort the bombers in, and win or lose on the sides ability to escort the bombers in and out, or is it point based where the fast fighters can stealth in and pick off objects. Personally, and I will work to balance the design no matter what, but personally, a massive mission like this should hinge on an orchestrated effort to keep the bombers alive and get them to target, not let the swift fighters in to pick off the buildings. It was a factor in the last event, the attack planes were the deciding element, a major event like this shouldn't come down to who got the fighters in, but who can get the bombers in. IMHO, I haven't talked to Brooke at all about this design, just tossing in options to consider. This event should hinge on getting a coordinated effort across 4 battles, 3 hours each, and get the bombers to target. It's too easy to get the attack planes in from the sidelines, it's incredibly difficult to coordinate a successful escorted bomber campaign. If someone is going to invest 12 hours or a fraction of that for an event of this caliber, it should be worth the effort. Again, My Humble Opinion. (Ok I know, I see you guys out there laughing at Humble)