The Russo brothers were "convinced" they'd never return to the Marvel Cinematic Universe (MCU).

Siblings Joe and Anthony Russo are returning to the franchise to direct 'Avengers: Doomsday' and 'Avengers: Secret Wars', but it wasn't in their original plans as they thought they were ready for new challenges after helming 2019's 'Avengers: Endgame', their fourth film in the comic book world.

Joe told RadioTimes.com: "I mean, we were very tired after those last movies, we did four of them in seven years. It was a lot of work, and we were asked a few times, and we said no."

Anthony added: "The work that we'd done in the MCU was really among the most fulfilling of our lives.

"We were able to tell a four movie narrative arc that was very interconnected and built... you know, each sort of expression built on the previous."

But the directors felt "compelled" to return after finding a new story to tell in the MCU.

Anthony said: "We're excited to come back now, because we've been away for a bit, we're excited to sort of pick that narrative through line up again that we were carrying, and find a new way to push it forward with everything that's transpired since that point in time.

"It's a challenge, but it's one that we think we finally lit on an idea that we felt really made it work... really made it worth us coming back to tell another story, so we can't talk about it in much detail sorry, but we're excited."

Joe added: "We finally, as Ant said, found a story that really excited us, [that] we felt compelled to tell over other stories that we've been working on, and so now we're back."

And Anthony teased that fans won't be disappointed.

He said: "If you like the work that we had done previously in the MCU, I think you should be excited for what we've got coming."

'Doomsday' will see Robert Downey Jr. - whose previous MCU character, Tony Stark/ Iron Man was killed off in 'Avengers: Endgame' - return to the franchise as the villain Doctor Doom, and the Russos recently told how he has been working hard on developing his new alter ego.

Speaking to 'Entertainment Tonight', Joe said: "It's a very intense process developing the character. He's so immersed in it. He is so dialed in."

Anthony added: "That's the kind of artist he is. That's the kind of actor he is."

Joe continued: "[Downey's] writing backstory, costume ideas … I think he just loves really rich three-dimensional characters and I think he sees a real opportunity here with that character."