Finneas O’Connell has revealed he and sister Billie Eilish wrote the theme song to the new James Bond film in the bunk beds of the tour bus.

The duo have written the title track for No Time To Die, which will be released later this year and will be Daniel Craig’s final outing as the spy with the licence to kill.

The track, which is sung by Eilish, was released last week and will be performed for the first time at the Brit Awards on Tuesday, alongside Hans Zimmer and Johnny Marr.

At 18, Eilish is the youngest artist to record a Bond theme song.

O’Connell, 22, told GQ Hype: “We wrote No Time to Die on a tour bus. Specifically, in the bunks of our tour bus.

“We were given the first 20 pages of the script. I guess that’s up to the point when the song comes in during the movie, right? That’s how all the Bond films open up.

“So, we were able to read the first 20 pages, which was obviously incredible.

“It gave us such a good steer and such insight into where the song would fall, and the tone.

“It makes it easier than having to write the whole song based on the entire movie; or in fact none of the movie.

“Billie and I wrote the song, recorded the demo, sent it to them and then we finished it in London with Hans Zimmer doing the orchestral arrangements and also Johnny Marr from The Smiths.

“I mean, James Bond? Hans Zimmer? Johnny Marr? Mind blowing.”

O’Connell said even after they wrote the song there was no guarantee it would be used in the film, adding: “Well, it’s not like Barbara (Broccoli, the producer) calls you up and you are asked to write a James Bond song.

“You write the song and then they tell you whether it’s going to work. It’s not yours until it’s approved for the movie, in some ways.

“I get it; and it adds to a certain amount of, let’s say, excitement. And pressure. I would be the same: I would never, as a director, say to an artist that they can write whatever they like for my 100 million dollar movie.

“It’s more, ‘Well if you write the perfect song, it’s yours.” Luckily, she liked our ideas.”

Asked if Americans are passionate about the British franchise, he said: “I think people in the States care about Bond now. Okay, maybe specifically Billie and I really care about Bond. We love it. Always have done.

“Bond is the ultimate franchise. What Barbara has done with Daniel Craig and the past few movies; it’s incredible. Have you ever met Broccoli? She’s not as you might imagine.

“She’s not interested in making a wrong move with her movies, if you know what I am saying. We were blown away to be given a shot and truly respect her vision for the whole package.”

GQ Hype (Luke Fenstemaker)

Discussing if they considered changing the title, as Sam Smith did with his Oscar-winning song for Spectre, Writing’s On The Wall, he said: “I always think with a James Bond song, it should be the title.

“I know there’s the Jack White one, Another Way To Die, but I couldn’t write a song called, for example, Quantum of Solace. I couldn’t!

“It’s a really cool title of a Bond movie, but a really hard song title to write to.

“Yeah, we lucked out because No Time to Die is a great song title. Unlike Quantum of Solace. Or Casino Royale. Not very evocative song titles.

“Although so much of Bond gets over-analysed to a certain extent.

“I know he’s part of your heritage over in Britain, intertwined with the culture and so on – but also, James Bond is just the coolest fictional character of all time. Right? And all those flaws he has, they just enrich the character. They make Bond more human.

“I think it was Skyfall where they really tested him; where we really got to see Craig’s Bond in full flight. It was astonishing. And ultimately that’s all story telling; so James Bond was a perfect brief for me and Billie in many ways. The brief of our lives.”

The full interview in GQ Hype is out now.