Most Christians don’t want to acknowledge that Jesus promised to return before His own generation had completely passed away. However, those with less faith in Him are not shy about doing so. Generally speaking, that’s who I’m referencing in the list below.
There is no reasonable basis for doubt that Jesus and His apostles expected the Second Coming (aka the coming of the kingdom of God, the day of the Lord, etc.) before the end of what we call the first century AD. Here are some people who lay out the biblical case on this issue and make the answer clear.
Ed Babinski – start with this post.
Thom Stark – start with this post. (It’s the same post as above, but from there you’ll be directed separately for Ed’s view and Thom’s view. Note also that when I present Thom’s view I’m doing so in the context of reviewing a book he wrote, and therefore you’re being directed to the post about his chapter on the coming of the kingdom of God.)
Bart Ehrman – see Chapter 9 “Jesus the Apocalyptic Prophet” in his 2012 book Did Jesus Exist? The Historical Argument for Jesus of Nazareth. For this chapter, Ehrman says he relied heavily on his previous book Jesus: Apocalyptic Prophet of the New Millennium (1999), so I suppose you could find the same argument there.
Albert Schweitzer – Schweitzer is widely known among Bible scholars for his defining 1910 book The Quest of the Historical Jesus, in which he describes Jesus as a failed apocalyptic preacher. To some degree Ehrman is simply repeating this argument, and is quite willing to say so.
Of course, none of the people above believe (believed) that the kingdom came when Jesus said it did. However, it did. Just when He said it would. See Jesus Christ Has Already Come Again.