Jesus hand chose the first 12 Apostles while he was on earth. But that did not limit Apostleship to only them. In the bible Apostles are missionaries who were entrusted by Holy Spirit to go out and spread the message of Christianity. So Paul was an Apostle, he definately fulfilled his missionary assignment to the Gentiles. As some already pointed out on this thread, the other Apostles were a little lacking in this work Jesus gave them. He told them to go to the Gentiles (Matthew 28:19,20) but they did not go. Later, Christ intervened by giving Peter a vision that helped him to see that they were supposed to preach the gospel now to the nations (gentiles) too.
Romans 11:13 (Paul speaking)
Iam talking to you Gentiles. Inasmuch as Iam the apostle to the Gentiles, I make much of my ministry
1 Corinthians 12:27-29
27 Now you are the body of Christ, and each one of you is a part of it. 28 And in the church God has appointed first of all apostles, second prophets, third teachers, then workers of miracles, also those having gifts of healing, those able to help others, those with gifts of administration, and those speaking in different kinds of tongues. 29 Are all apostles? Are all prophets? Are all teachers? Do all work miracles?
* Since there is not a limit placed on the number of teachers, prophets, miracle workers or others in the church and Paul called himself an Apostle, we can discern from this that the number of Apostles was not limited to the original 12. Again, Apostles were missionaries. Peace, Lilly