Early SBRs can slow the Axis war machine if America engages in them, but America also only has the two bombers on Round 1. That’s 7 IPC usually. Italy has 23 IPC after Round 1 and Germany has 45 IPC after Round 1. I don’t see 7 IPC being a huge impact in comparison to the 68 IPC the Europeans have. It’s barely more than 10%.
If America goes heavy bombers then they are also not going fleet (meaning Japan owns the Pacific and America only gets 1 NO, England none and Russia probably only 1 and Japan gets 3, Italy gets 2 and Germany gets 2.) If America does not go fleet, then their SBR campaign is offset by the excess IPC the axis are pulling in that the Allies are not. (You need a lot of bombers to dry up the extra 25 IPC the axis will be pulling (possibly 30 IPC if Germany gets the 3rd NO as well) and make up for the cost of lost bombers AND on top of all that, do damage to the enemy.)
I just don’t see it being a winning move.
If a bomber does 1.5 IPC more damage than it costs to replace the bomber in the time it is expected the bomber to be shot down and the axis have 25 IPC in NOs MORE than the Allies (That is 35 IPC in NOs - Allies 10 IPC in NOs = 25 IPC in NOs in excess of the Allies) then it you would need 17 bombers to show a profit. (16.67 to negate the extra income the Axis are getting and .23 to enter into positive territory.)
If you had 17 Bombers, doing 3.5 IPC each (assuming 3 are shot down by AA Guns before they can fire) you would have:
14 * 3.5 = 49 IPC in damage to the Axis.
Since this, invariably, means in Europe only, you are really limited to 32 IPC in maximum damage you can do. (12 to Italy, 20 to Germany.)
IF the allies do not go into the Pacific and focus on a bomber campaign you have the following:
Axis 7 to 8 NOs
Allies 2 NOs
Net Gain (Axis): +25 IPC
Maximal Damage to Axis Industry: 32 IPC (12 for Italy, 20 for Germany.)
Average Bomber Damage: 3.5 IPC
Average Damage to Bomber: 2 IPC
Net Benefit of Bombing Run: 1.5 IPC
To negate 25 IPC, the Allies need 25/1.5 = 16.67 Bombers
Most likely scenario of 17 Bombers attacking in SBR: 3 Bombers shot down to AA Fire
17 Bombers - 3 Bombers lost to AA Guns = 14 Bombers left
14 Bombers, doing an average of 3.5 IPC each = 49 IPC in combined damage
Maximum Damage Italy/Germany can take: 32 IPC. Damage from Bombers: 49 IPC. 17 IPC in bomber damage wasted.