Okay, lets see what a P-39N would do. 120 gals internal plus a drop tank of 110 gals = 230 gals. Takeoff and climb to 5000' reserve is 20 gals so now we're down to 210 gals available. A P-39N burned 62gph at 25000' at 2600rpm max continuous power so we have a theoretical 3.4 hours to escort those bombers. Reduce that by 15 minutes combat and a 20 minute landing reserve so now we have 2.8 hours of actual escort time. We're flying faster than the bombers (230mph) so we're weaving to keep from getting ahead of them. So really we're covering the same ground as the bombers but flying faster. The bombers will cover 640 miles in that 2.8 hours and we're going out AND back so our roughly estimated combat radius is about 320 miles. About 50 miles less than a Thunderbolt with drop tanks. If we can cruise at less than 2600rpm maximum cruise (not recommended over Europe) then we can go farther. All hail the mighty Thunderbolt.
If you want to develop a Kentucky windage scale model please consider the following:
Start and warm up engines, taxi to transient, take off in pairs and orbit as elements build into flights and into squadrons and finally assemble on group leader for climb. All of the preliminary fuel consumption will be near Max continuous power (Normal @ 39-40"Hg) with approximately 1-2 minutes of Take Off Power ~MP. Tanks would be switched to external tank (75 gallon CL) and climb at Max continuous until 25000 feet. The drag on the P-39 is at maximum while the tank is full. Time for warm up, Take Off, assembly and climb to 25000 feet ~ 25 minutes with total fuel consumption of approximately 30-35 gallons (including 10-15 internal, 20 external) leaving 105-110 internal and 55 external. Remove the fuel required for loiter/reserve for let down and landing - another ~ 10 gallons. So, at start of Cruise for R/V point you start with 120-25=95 gallons of internal fuel plus 55 external.
For mission planning the Combat Radius must account for a.) drop tanks for combat leaving only internal fuel, b.) 5 minutes of Combat Power, c.) 15 minutes of Military Power at an average of ~2.5 to 3X Cruise Power consumption. That translates to burning 20+ gallons of your remaining (120-15-25) for takeoff/formation assy) - leaving you with 60 gallons to get home and land with reserve.
Before going back to your escort cruise scenario, let's pause:
AT optimal cruise power setting with low RPM/MP the Allison at 10K was ~ 60+ gallons per hour and the cruise speed with external tank was 20-30mph less than in clean mode. If the optimal cruise speed of the P-39 was the same as the turbo/supercharged P-38 at 25000 feet (doubtful) then 250mph TAS less 30mph=220 mph TAS for external 75 gallon tank (with 55 remaining internally) --------> less than 1 hour and approximately 220 miles from start of escort cruise at 25000 feet (optimistic) before dropping empty tanks.
If combat occurs under above scenario, then the 60 gallons at 250 mph (now clean) gets you back over the channel in the 1 hour of optimal fuel consumption.
The Brequet equations will yield a more optimistic range based on ratio of L/D and the fact that Induced Drag will decline as the weight fraction of fuel decreases over time.
Two points - I have never seen equivalent Range tests performed at Eglin for P-39 (as exist for P-51 and P-47) but I know that the Power/Thrust available per pound of fuel consumed for the P-39 Allison at 25K is much less than the P-51. The Mustang Packard consumed about the same fuel per hour but the cruise speed for max mileage at both 10,000 and 25,000 feet were so much higher because of the superior L/D of the Mustang. The cruise speed of the P-51B/D was about 290-305 mph TAS at 25000 feet for optimal range - a FAR better combat state than either P-39 or P-38.
Second point - I am Not positive about the aggregate estimates of internal fuel consumption for the narrative for P-39 and substituted a P-51B narrative. Would be delighted of you can point me to a source that clarifies operational data for the P-39 for warm up, take off, climb and cruise (clean and w/external 75 gallon tank) at 25K