Early covers (especially those done under the Virgin Killer name in the very very early days) were punk songs for the most part, including selections by the Ramones, and The Dead Boys. By 1980, they had recruited two guitarists (names forgotten over time) and the setlist included songs by Motorhead ("Overkill", half of the Ace of Spades album, and others), Judas Priest ("Tyrant" was their closer for a while), and later Iron Maiden and Riot, as well as a smattering of punk covers, which were played with extra distortion, intensity, and concentration on riffs, thus marking Overkill as the first thrash band.
In 1981, Dan Spitz (later of Anthrax) left the band and Rich Conte entered, along with Bobby G. At this point, they started writing originals, including "Grave Robbers" (later renamed to "Raise the Dead"), "Overkill", and "Unleash the Beast Within". This is the first thrash metal original, predating James Hetfield and Hugh Tanner's "Hit the Lights" by several months.
More originals would follow, including "Death Rider" (1981) and "Rotten to the Core" (1982) - somewhere around this time Rich left and Bobby remained the sole guitarist. The band became a staple at New York and New Jersey clubs around 1982, and D.D. gave Bobby the name "Blitz" due to his over-the-top lifestyle. (It actually earned Bobby an ejection from the band for a few days in 1983!) Around this time, the green logo was adopted - it was specifically chosen to stand out on a poster with lots of red logos of other bands on the bill.
The lineup would stay consistent until 1987, when "Rat" Skates left the band. It is rumoured that he either became a truck driver, or died in a bizarre gardening accident. In any case, he was replaced by Mark Archibole for a few gigs, and then on a permanent basis Sid Falck, previously of Paul Di'Anno's Battlezone. (Bobby Blitz once said that the only former member he misses is co-founder Rat Skates.) Then, in 1990, Bobby G left and was replaced by Rob Cannavino and Merritt Gant (Faith or Fear).
This lineup lasted until 1992, when Tim Mallare came in on drums. Rob and Merritt both left the band in 1995 (Rob became a motorcycle racer, and Merritt formed several other bands), being replaced by Joe Comeau (Liege Lord) and Sebastian Marino. Joe left in 1999 to become the vocalist for Annihilator, and the Coverkill album was recorded with new guitarist Dave Linsk. Sebastian left to spend more time with family, so Linsk played all the guitar parts on 2000's Bloodletting album, and Derek Tailor joined for the subsequent tour, and was also featured on the band's latest album, Killbox 13.