The answer varies from time to time, place to place and airplane to ariplane. In the beginning of the war B -17 fighters were sent without fighter escorts while in the Pacific at the and of the war there was little resistance on the part of the Japanese to the death that rained down from above. Then you have the Lutwaffe in the battle of Britain which was utterly destroyed while at the end of the war in the English Gloster Meteor protected home skies and I have not heard of one being shot down. If you want a more definitive answer search for individual squadrons of fighter and bomber planes and the casualties suffered by them and compare them, while you will not cover a fraction of the total air forces you can get an idea of what went on in the skies.