Weren't there external hardpoints in early B-17's ? Maybe they were needed to achieve the maximum loadout ?
Edit: copy&paste from Boeings site
Span: 103 feet 9 inches (B-17G)
Length: 74 feet 9 inches (B-17G)
Gross weight: 65,000 pounds (B-17G)
Top speed: 287 mph (B-17G)
Cruising speed: 150 mph (B-17G)
Range: 3,750 miles (B-17G)
Ceiling: 35,600 feet (B-17G)
Power: Four 1,200-horsepower Wright R-1820-97 engines (B-17G)
Accommodation: 2 pilots, bombardier, radio-operator, 5 gunners (B-17G)
Armament: 11 to 13 machine guns,
20,000-pound bomb load (B-17G)
EditII:
http://www.b17.org/flying_fortress.html says "Bomb Load: 2,600 - 8,000 lbs.(long range) up to 17,600 lbs.(short range)"
Guess it would be harder to code way that max bomb load depends of fuel load.