The Black Eyed Peas frontman wowed viewers when he sung the track with McFall during the final of this year's series of the talent contest, in which she eventually finished second.
Following the success of the performance - which propelled the original to number 3 in the UK chart - Will told a fan on Twitter in July that a studio recording with Leah would be 'coming soon'.
However, it was kept under wraps until this week, when it was exclusively made available on the 'Now! That's What I Call Music 86' compilation.
Leah isn't credited on the sleeve of the album, but her inclusion was confirmed by the official Now! Twitter page, who wrote today: "So much love for @leahmcfallmusic's version of 'Bang Bang' with @iamwill"
The 24-year-old singer added: "Yip. Bang Bang - the version with me and @iamwill has been released on the Now86 album!! yay!!"
'Bang Bang' is Leah's first release since The Voice concluded. Will recently teased the progress of her debut album - expected next year - saying: "I’m producing and writing it. It’s the same way I produce a Black Eyed Peas or Fergie album, it’s getting that level of attention to detail.
"I’m working as hard on it as I have done on all the other projects I’m most passionate about. I haven’t done that before with someone I’ve just met."
Leah previously reached number 8 in the official singles chart with a recording of her cover of 'I Will Survive' from The Voice's live shows.
Now 86 is available to buy both in stores and online now, while 'Bang Bang' can be purchased individually from download stores such as iTunes and Amazon.