Even for victory credits that statistic is clearly wrong among all a/c. The (aerial only) victory credits by general type of a/c and theater in USAAF were as follows:
Theater/heavy bombers/med and light bombers/fighter
ETO/6098/103/7422
MTO/3178/510/3300
Pacific (means 7th AF)/183/22/370
Far East AF (most of rest of PTO)/1344/231/2709
CBI/283/46/847
20th AF/914/0/80
Alaska/29/9/34
source: USAAF Statistics Digest, which adds up to:
Total/12029/921/14762
So, total bomber credits rivaled fighter credits, but probably less than half of those were credited to B-17's, I don't know an exact stat for that.
BUT, if the question is about how many enemy a/c *really* shot down, the proportion by bombers would be much lower. Bomber credits were consistently vastly exaggerated. Drgondog has said that he can correlate 8th AF fighter credits pretty well to German losses assuming only 10% of the simultaneous bomber credits are valid, I'm sure he can comment more. When I've looked at more micro situations where the bomber credits can be directly correlated to enemy fighter losses (in absence of US fighters, or where the details allow one to see the exact cause):
-Japanese fighter losses to 20th AF B-29's in Nov '44-Feb '45 unescorted operations were around 25% of the US credits, but quite a few of those losses were suicide rammers, hard to mistake those losses.
-B-29's were credited with 27 MiG-15's over Korea; studying each case, and Soviet accounts are known for each, the real score was probably 3 MiG-15's.
-See "Revenge of the Red Raiders"by Gaylor et al, about the 22nd BG in the Pacific. It's well correlated with Japanese accounts which I've also studied separately. Per the USAAF overall stats the mediums and lights were credited with 23 enemy a/c in the air in May and June 1942, that was only the (then B-26 equipped) 22nd plus the B-25 equipped 3rd BG at that time. The Tainan AG actually lost only 2 Zeroes to those two units in that period, one to each, and they were the only Japanese fighter unit engaged. Several others were damaged though.
USAAF fighter credits OTOH varied a lot in accuracy. Back to New Guinea 1942 , for example the 8th FG in its April-May 1942 tour was credited with 32 Zeroes; the Tainan AG's actual losses were around 10 or 12, and again they were the only Japanese fighter unit present at the time. The losses are quite clear in their reports including every combat recorded on the US side, but the causes are uncertain in a few cases.
But in later periods of WWII USAAF fighter credits were sometimes considerably more accurate than that. It's hard to generalize about USAAF fighter credit accuracy, whereas it can be reasonably generalized to say that bomber credits (USAAF's or anyone else's AFAIK) seldom bore much relationship to reality.
Joe