D.45 was the brainchild of Dave Mustaine (of Megadeth; guitar) and Lee Ving (of Fear; vocals). The one-off side project was intended to allow Dave Mustaine to expand on his own without the constraints of his other band Megadeth. Recruiting Kelly Lemieux (of Shrine; bass) and Jimmy DeGrasso (ex-Suicidal Tendencies/ex-Megadeth/ex-Y & T; drums) they went about publishing their debut (and only) album, The Craving (1996). The album was exactly as expected, a Megadeth spin-off with Megadeth guitar playing and Megadeth vocals. The difference is it had more of a punkish feeling to it, but only a bit. The project went silent there-after, with each performer returning to their respective bands of origin.
Almost a decade later, while 'Capitol Records' was releasing remastered versions of Megadeth's back catalog, during the remastering process, it was realized that the vocal track and harmonica track were missing from the re-release of The Craving. These were suspicious omissions since Ving's vocals and harmonica work had been a drastic departure from the sound of Megadeth, and reasons why it was believed fans initially rejected the original album; Mustaine chose to replace the vocals to appease the Megadeth audience and hopefully have better sales. So, to complete the remaster, Mustaine chose to sing Ving's vocals himself and simulate the harmonica parts on the guitar.
Footnote: There was also an EP released called Voices that featured tracks that would soon after appear on the full length album.