Backstreet Boys' AJ McLean Reveals How Drake's 40th Birthday Show Came Together

Drake had two dream acts in mind for his 40th birthday bash ... and lucky for him, the Backstreet Boys were available to make his wish come true.

AJ McLean is spilling the beans on how the boy band's surprise performance at the rapper's lavish New York City party came together ... revealing Drake's manager reached out with a pretty specific request.

AJ says Drake only wanted one of two acts to perform at his birthday celebration ... either Sade or the Backstreet Boys.

The group still had to get everyone on board ... and AJ admits he was a little unsure about Kevin Richardson, who usually switches into full-on football dad mode when the band isn't working.

But AJ had a feeling Kevin would say yes ... joking his two sons, who are huge Drake fans, would've been furious if their dad passed up the opportunity.

And it sounds like Kevin made the most of the gig ... with AJ joking he was practically glued to Drake the entire night.

"Anytime Drake moved anywhere, Kevin was right there," AJ said, laughing about his bandmate's newfound friendship with the birthday boy.

As TMZ previously reported ... Drake was absolutely beside himself when the Backstreet Boys surprised him at his 40th birthday bash Friday at Cipriani 42nd Street, even joining the group to belt out their hit "I Want It That Way."

The party was no ordinary birthday celebration either ... with more than 400 guests, luxury Chanel and Hermès giveaways, and a game offering cash prizes as high as $1 million.