The eschatological discourse in Matthew 24 (derived from Mark 13 and paralleled in Luke 21) concerns the destruction of Jerusalem and what was to shortly follow it. Mark presents both the Jerusalem invasion and the coming of the Son of Man in judgment as closely connected, which reflects the time when the gospel was likely written (i.e. shortly before or after AD 70). Some time has passed by the time Matthew was written (most likely around AD 80-100), and thus Matthew 24:27-25:30 adds a series of parables explaining the apparent "delay" (missing in Mark), and rephrases the disciples' question in Matthew 24:3 to explicitly refer to the "end of the world" because the reply to the original question in Mark simply assumed that the end would come along with the destruction of Jerusalem. Thus Matthew acknowledges the apparent delay of the parousia, and yet indicates that it would not be for very long -- for it would still occur in the lifetime of those who heard Jesus (cf. Matthew 10:23, 16:28, 24:34).
The actual wording in Matthew 24:14 is: "And this gospel of the kingdom will be preached throughout the whole world, as a testimony to the nations, and then the end will come". The "end" referred to here is the same "end of the world" mentioned in the disciples' question in v. 3. Just as the disciples' question was reshaped by the author of Matthew to reflect a separate time for the "end of the world", so is v. 14 a Matthean redaction of the original phrasing in Mark 13:10: "And the gospel must first be preached to all nations". Note that the original wording did not refer to an "end", and it was followed by a description of the presecution that Christians endured for their efforts. As mentioned above, the "end" was not thought to be a time in the distant future (like, say, the 21st century), but within the lifetime of those who heard Jesus. The author of Matthew also had no concept of "inhabited earth" (oikoumené) being any larger than what was known in his own day (e.g. the Roman Empire). In this connection, Paul also believed that in his day the gospel had already been spread throughout the whole inhabited earth:
"Not everyone, of course, listens to the Good News. As Isaiah says, 'Lord, how many believed what we proclaimed?' So faith comes from what is preached, and what is preached comes from the word of Christ. Let me put the question: Is it possible that they did not hear? Indeed, they did; in the words of the psalm, 'Their voice has gone out through all the earth, and their message to the ends of the world' " (Romans 10:16-18).
In the same epistle, Paul referred to his "apostolic mission to preach the obedience of faith to all pagan nations (en pasin tois ethnesin) in honor of his name ... your faith is spoken of all over the world (en holó tó kosmó)" (1:5, 8). As early as 1 Thessalonians, Paul said that "the word of the Lord started to spread -- and not only throughout Macedonia and Achaia, for the news of your faith has spread everywhere (en panti topó), we do not need to tell other people about it" (1:8). Similarly, Paul writes in Colossians 1:5, 23 that "the Good News which has reached you is spreading all over the world (en panti tó kosmó) ... [and] has been preached to all creation under heaven (en pasé ktisei té hupo ton ouranon)". These statements were made even tho clearly the Christian gospel had not yet reached the frigid reaches of Siberia, the plains of the Americas, the isles in Polynesia, or the aborigines of Australia. Similarly, when Acts 2:5 says that "there were devout men living in Jerusalem from every nation under heaven (apo pantos ethnous tón hupo ton ouranon)," the author certainly didn't mean to say that there were Chinese, Sioux Indians, Eskimos, and aborigines living in Jerusalem in AD 33. Thus, there is no reason to interpret Matthew 24:14 as demanding a greater preaching area than was covered by first-century Christians, for expressions like "all the inhabited world" were commonly used to refer to what was already accomplished or being accomplished in the time of the apostles.
And since Matthew expected the end to come very soon in the first century or early second century (cf. Matthew 10:23, 16:28, 24:34), the reference was certainly not to a future preaching work many centuries later.