Most of this data comes from the 8th Air Force April 1945 monthly report, which includes 2 tables of effort, one for the B-17 and one for the B-24, it notes casualty figures are not finalised as some reports still have to come in from aircraft that landed in Europe, not back in Britain.
For example it seems the total number of heavy bombers written off after a combat sortie for the war is 1,557 versus 1,562 given in the April report.
In addition the sorties are for stated to be bombing missions only, however the final 8th Air Force report has slightly higher sortie figures, 330,866 versus 328,590 in the April report, so if a final report can be found that splits B-17 and B-24 effort the numbers will vary slightly from the ones presented here.
In terms of numbers the B-17 accounts for 68.9% of all 8th Air Force heavy bomber sorties, 69.2% of such sorties that entered contested airspace (credit sortie) and 70.2% of sorties reported as bombing (effective sortie). The average B-17 bomb load for the war was 5,139.4 pounds versus 5,329.95 pounds for the B-24. It means the B-17 dropped 69.45% of the bombs dropped by the 8th Air Force heavy bombers.
As can be seen by the sortie percentages it seems the B-24 was more vulnerable at about every stage to factors causing the sortie to be abandoned. One reason seems to be a different use of spare sorties, aircraft taking off used to replace any aborting sorties, so formation left Britain at full strength. Some 4,621 B-17 sorties landed classified as "unused spares" versus 302 B-24, which probably accounts for some of the later differences in sortie abort rates.
Having said that some 91.4% of B-17 credit sorties were classified as effective, versus 87.1% for the B-24. So while 68.9% of heavy bomber sorties were B-17s they accounted for 61.95% of mechanical, 58.7% of weather and 61.45% of other reason aborted sorties. The B-17 was the more reliable as used by the 8th Air Force.
For 1944 and the first 4 months of 1945 the B-24 had an accident rate on operations of 0.3 per 100 take offs and 1.01 accidents per 1,000 hours of non-operational flying, versus 0.22 and 0.97 respectively for the B-17.
For the entire war 1.58% of B-17 credit sorties were Missing In Action, versus 1.2% of B-24, partly this is the result of the losses in 1943 (nearly a quarter of total B-17 losses) but if you simply use the 1944 results the loss figures actually increase, to 1.61% for the B-17 and 1.3% for the B-24. So unless the B-24 was deliberately and regularly being sent to less heavily defended targets the figures indicate you were more likely to come home in a B-24.
Another interesting point about losses is for every 4.15 B-17 listed as missing another was written off after a combat sortie, for the B-24 it is 3 missing for each write off.
The loss figures at first examination seem to be saying the opposite to the anecdotal evidence, the figures are reporting the B-24 was the more survivable, taking fewer losses and making it back with heavier damage. Alternatively in the damaged situation it could be the B-24 was more prone to taking significant damage in a crash landing. The figures indicate this was the case, 0.5% of B-17 credit sorties were written off, versus 0.6% of B-24, some 20% more than the B-17 but the B-17 missing to write off ratio is approaching 40% more than the B-24.
So short of a systematic bias in target selection the B-24 was more likely to abandon the mission and more likely to crash but more likely to bring you back, even when damaged, even when accounting for it taking more damage in a crash landing.
The USAAF had 4 categories of damage, A, repairable within 36 hours by the unit, AC, more than 36 hours to repair and needing non unit resources like a sub depot, B requiring a full Repair Depot to fix E write off.
For the war, using 293,025 credit sorties, there were 43,601 category A, 13,893 category AC, 593 category B and 1,557 category E, or 20.4% of all credit sorties came back damaged enough to be classified as damaged. By year it was 27.4% in 1942, 30.9% in 1943, 21.1% in 1944 and 15.6% in 1945.
The top 4 months were all in 1943 and are the only figures above 40%, being 48.4% in January, 43.9% in July, 42.4% in August and 41.7% in October. Being percentages these figures are prone to big shifts when there are only a small number of sorties, in 1943 there were 22,099 heavy bomber credit sorties, versus 188,036 in 1944 and 81,912 in the first 4 months of 1945. There were only 279 credit sorties in January 1943, but over 2,000 for the other 3 months listed above.
The night bombers consistently reported higher MIA figures than the day bombers for 1945, as a percentage of sorties, but it seems the number damaged, including to write off stage, was around 2.8% of sorties despatched. Versus 15.6% for the 8th Air Force credit sorties, which were about 90% of airborne sorties. The extra maintenance requirements from consistently having several times the number of damaged aircraft would have been significant.