There's a direct wartime comparison of fuel consumption between a B-17E and a B-24D at WWII Aircraft Performance:
The B-17's range advantage is remarkable. Partial results from the above test:
At 10,000 feet and 60,000 lbs initial gross weight
Fuel load | Engine power | B-17E range | B-24D range |
1500 gal | 70% | 1405 miles | 870 miles |
| 60% | 1680 | 1400 |
| 50% | Not given | 1575 |
2000 gal | 70% | 1892 | 1165 |
| 60% | 2236 | 1880 |
| 50% | Not given | 2150 |
2500 gal | 70% | 2400 | 1460 |
| 60% | 2910 | 2380 |
| 50% | Not given | 2735 |
The B-17G is either lower drag than the B-24D (despite the supposed performance advantage of the Davis wing) or the R-1830 has developed a heavy drinking problem compared to the R-1820.
Results from the above test were also given for both types of aircraft at the same engine power and fuel loads (with cruising speeds given as well). At the same hp per engine the B-17 has a range advantage of anywhere from about 6% through to 42%!
Also, if the B-17 is galloping along at 660 hp per engine and 202-218 mph, the B-24D only just betters the B-17's range but bumbling along at 550 hp per engine. That also puts the B-24D down in speed by 12-20 mph.