Pretty much all of them were escorted... Typically, the fighters would escort the bombers to the limit of ther range, then leave them, then pick them up again on the return leg... It was only later on that the Mustangs were able to escort them to the deep targets and back.
The Luftwaffe tactics of the day were to intercept the bombers as early as possible with a token attack to get the escorts (P-38s and P-47s usually) to drop their tanks in order to reduce the already limited range of the escorts.