A lot of the "mission creep" started fairly early. In part because the B-25 and the B-26 were in the design stage before WW II started in Poland.
"on August 10, 1939, the Army issued a contract for 201 Model 179s under the designation B-26 "
The plane was ordered off the drawing board with no prototype.
Combat experience by the French and British showed that the original set up as designed had several flaws or gaps in capability. (no armor or self sealing tanks for one thing)
"Although the first B-26 had yet to fly, orders for 139 B-26As with self-sealing tanks and armor were issued on September 16. Further orders for 719 B-26Bs on September 28, 1940 brought the total B-26 order to 1131 aircraft."
first Flight by a B-26 was not until November 25, 1940 and the Army got the first ones on February 22, 1941.
One the early aircraft there was single .30 cal in the tip of the nose, the two .50s in the top turret (first power turret on a US bomber) a single .30 cal out the bottom and a single .50 in the tail. Crew was five men.
Crew weight, as given in the weight and loading charts in the March 1942 manual, was rather optimistic. 950lbs for all five men including parachutes, WHile the pilot, copilot and radio operator could all weigh 200lbs each (with parachute) the navigator was a svelte 180lbs and the gunner a mere 170lbs (again with parachute).
Ammo for the guns was 600 rounds for each of the .30 cal guns and 200 rounds for each of the .50s.
At the rear of the plane the turret gunner was expected to get out of the turret and lay on his belly to operate the .30 cal tunnel gun and then bounce up and climb into the turret if the threat should get into the upper hemisphere of the plane.
Radio operator (or co-pilot?) went through the bomb bay and into the the tail to man the tail gun (?) hardly an ideal situation in combat.
The 326mph listed in the manual for the early planes was done at 26,734 lbs design weight. But normal gross weight as operated was 28,706 lbs which included only four 500lb bombs and just 465 gallons of fuel.
for those who are interested the manual for the early planes is here:
http://www.wwiiaircraftperformance.org/B-26/B-26_Operation_and_Flight_Instructions.pdf
with lots of charts and tables.
The high performance of the early B-26 was somewhat of an illusion as even without chin guns and some of the other add ons it's combat weight had gone up thousands of pounds.
The size of the bomb bay was not governed by the requirement to carry a torpedo but by the desire of the AAF to carry thirty 100lb bombs.
Some speed was lost by the deletion of the propeller spinners and the larger air intakes (to house dust/dirt filters) and perhaps a large oil cooler intake.
there are 3 basic B-26s, the original small wing, the big wing and the big wing with tilt.