St. Paul the Apostle & Author of Hebrews

There is overwhelming evidence to support the fact that the Apostle Paul wrote the Epistle to the Hebrews.

The Epistle’s location. It's located right at the end of the other Pauline Epistles and neatly fits right at the end.
If Paul didn't write Hebrews then you can't say that he wrote half of the New Testament. Because not including Hebrews, there are 13 Pauline Epistles and 27 New Testament Book altogether. But if you include Hebrews as one of the Pauline Epistles, then that is a total of 14 Books written by Paul—more than half of the New Testament Books!
Moreover, if you include the Epistle to the Hebrews, there are exactly 100 Pauline Chapters in the Bible!
I have 5 Bibles (Cambridge KJV, Zondervan Gift & Award KJV, and Gideon's New Testament KJV Pocket Bible, JVI KJV, & Christian Duplications audio cassette Bible) that all credit St. Paul with the penmanship of the Book of Hebrews.
By the grace of God in Christ Jesus, I have a reproduction of the 1611 King James Version by Oxford. The 1611 attributed the Epistle to the Hebrews to Paul: Apostle Paul’s name is in the heading of the letter.
Whoever wrote Hebrews had a profound knowledge and understanding concerning the Old Covenant. The Book of Hebrews compares and contrasts the Levitical Priesthood of the Old Covenant with the New Testament Priesthood of Jesus Christ. The Epistle also reveals that the Old Testament precepts shadowed the office which Christ has now fulfilled under the New Covenant. The Apostle Paul had exemplary understanding in both, because before he was saved he was a devote Pharisee. (cf. II Cor. 11:22; Gal. 1:13-14; Phil. 3:4-9) He said that he was "…an Hebrew of the Hebrews…" (Phil 3:5) Someone who was 'an Hebrew of the Hebrews' would certainly be someone you'd consider as the penman of Hebrews, "And unto the Jews I became as a Jew, that I might gain the Jews; to them that are under the law, as under the law, that I might gain them that are under the law" (I Cor. 9:20) Remember St. Paul became a born-again Christian and Hebrews is a Christian Epistle. And after his conversion he became the model for Christian zeal… "But by the grace of God I am what I am: and his grace which was bestowed upon me was not in vain; but I laboured more abundantly than they all: yet not I, but the grace of God which was with me." (I Corinthians 15:10) In the Pauline Epistles, the Apostle Paul quoted from the Old Testament Books more than any other New Testament Apostle. (Not even including Hebrews.) Man, but I don't really like these comparison things, Paul himself wrote under the inspiration of the Spirit, "…but they measuring themselves by themselves, and comparing themselves among themselves, are not wise." (I Cor. 10:12) So I'll stop here and realize we're all one Body in Christ and Christ is the Head. And by Christ all things exist and consist. In Jesus' Name, Amen.
The Epistle to the Hebrews is widely known as the greatest Book in the Biblical Canon on FAITH. And I have FAITH enough to believe the KJV’s title to the Epistle is correct, in that “Paul the Apostle” wrote it!
St. Paul also had a heart for the Jews. Even though he certainly gave his everything to the Apostleship to the Gentiles (Rom. 11:13), he still mentioned the Jews often in his writings and by reading the Book of Acts we also see that he preached to many Jews. Granted that he often used the Jewish theology as a pivot point to turn and preach the grace of Christ. But also you read in Romans 9:1-3, 11:14 that St. Paul had a great sense of compassion for the Jews. Therefore the Epistle to the Hebrews may have been penned by Paul out of the Holy Spirit's compassion.
We see a similar analogy concerning Mt. Sinai and the City of Jerusalem discussed in both St. Paul's Epistle to the Galatians (see Gal. 4:24-26) and Hebrews 12:18-24.
"For ye had compassion of me in my bonds, and took joyfully the spoiling of your goods, knowing in yourselves that ye have in heaven a better and an enduring substance." (Heb. 10:34) We see here 'the writer' mentions that had been in bonds, and that the Hebrew Church had apparently communicated with him at this time as well. It's a well know fact that throughout the New Testament Paul was frequently and often in bonds, chains, and imprisonments. There are scores of Scriptures that reveal the tremendous persecution which he endured for Christ's sake. (see II Cor. 11:21-33, 12:5-11) From Acts Chapter 21-28 we read of Paul's imprisonment. At the end of the Book of Acts (Chapter 28) Paul is finally in Rome. According to Church history he remained in Rome until his martyrdom under Roman Emperor Nero. The Book of Hebrews was very likely written from Rome. (Before the destruction of Herod's Temple. Because the holy Epistle counters Judaism.) He mentioned himself as “the prisoner of the Lord” in Eph. 4:1 and Philemon 1. And talks about his “bonds” in Philippians Chapter 1 verses 13, 14, 16.
He mentions Timothy in Hebrews 13:23. Timothy was one of Paul's fellow Christian labourers, he wrote 1^st and 2^nd Timothy to him, and also called him his son.
Habakkuk 2:4 is quoted in Hebrews 10:38. This verse is quoted also in two other places in the Bible (see Rom. 1:17; Gal. 3:11), both Pauline Epistles. "The just shall live by faith…" is a Pauline doctrine.
"labour of love" (I Thess. 1:3; Heb. 6:10) is a Pauline phrase.
There's no question that Apostle Paul wrote the Epistle to the Romans. In Romans 9:7 he quotes Genesis 21:12. Genesis 21:12 is also quoted in Hebrews 11:18. ¶ Again, Apostle Paul quotes Deuteronomy 32:35 in Romans 12:19. Deuteronomy 32:35 is also quoted in Hebrews 10:30.
He mentions Italy in Hebrews 13:24. Rome is a city in Italy. According to the Book of Acts, Paul lived in Rome because he appealed the false accusations against him to Caesar. He also wrote the Epistle to the Romans. Church history also tells us that our dear Apostle Paul was beheaded under the Roman Emperor Nero. He mentions “Caesar's household” in Phil. 4:22.
And finally we see the Apostle Paul's blessed salutation in Hebrews 13:25 "Grace be with you all. Amen." When we compare this salutation which is at the end of Hebrews, with the closing salutations[i] at the end of the rest of the Pauline Epistles: it is clear to see this was his writing. Paul even said that this particular salutation was a token of his writing. (cf. II Thess. 3:17,18) Other than Jesus Christ himself, Paul has the greatest grace preacher ever (just read Romans), therefore it makes perfect sense that he would close the Book of Hebrews by mentioning grace.