Contextually, Mt 24:14 was referring to events that were to take place leading up to the end of Jerusalem and the temple (that actually occurred in the 66-70 AD time). (Compare what was to happen afterwards: Mt 24:15-20.)
"Inhabited earth" (oikoumenē) in the NT generally refers to the area occupied by the Roman Empire or the then known world. (Compare e.g. Luke 2:1; Acts 11:28) And Paul indicated that he understood this to have already happened before the Roman-Jewish War of 66-70. (Rom 16:25, 26; Col 1:23)
That's not to say that Jesus' disciples weren't commanded to preach to the literal ends of the earth (Mt 28:19-20; Acts 1:8). But Mt 24:14 itself is not saying that, if you go strictly by the context. Although, I think a JW and others also, would argue for a dual fulfillment.
(Expanding out in the context, Mt 24:14 would also be limited in time to "this generation" in Mt 24:34.)
Bobcat