Prior to , all the members were actively involved in the entertainment industry: Feeldog debuted in 2012 as a member of Big Star. Euijin joined A.cian in 2014 under the name Lo-J but left the group in 2015 due to problems with the company, later debuting again in 2016 as a member of Bigflo. Daewon debuted as a member of Madtown in 2014 which later disbanded during the show. Ko Hojung debuted as a member of Hotshot in 2014. Jun joined U-KISS as the youngest member in 2014. In 2017, Chan debuted as a member of A.C.E, Kijung as a member of IM, and Marco as a member of Hot Blood Youth. Ji Hansol is a former SM Rookies trainee who left the company in 2017 to sign with J-FLO Entertainment and is set to debut in the upcoming boy band New Kidd. In July 2017, KBS announced their new survival show that would create male and female unit groups, with nine members each, among idols who had already debuted. The program aimed to give them a fair chance to demonstrate their talents that they might not have been able to showcase before. It premiered on October 28, 2017 and concluded on February 10, 2018. In November 2017, it was announced that the winning group will do promotions for seven months and if the winning group gained success, their contract will be extended up to twenty-five months. The winning group members had opportunity to participate in other activities with their original agencies. On February 21, 2018, it was revealed that the male group, Uni+ B, would hold their first fan-meeting on March 3 at. The tickets sold out within two hours of being on sale. On February 24's broadcast of The Unit's Special Show, the final nine male contestants that formed "Uni+ B" became UNB following the announcement.
2018–2019: Debut, ''Boyhood'', ''Black Heart'' and UNB Disbandment
On April 7, UNB officially made their debut with their extended play titled Boyhood. The EP featured double title tracks, "Feeling" and "Only One". They held their debut stage on You Hee-yeol's Sketchbook. On June 1, a dance battle video was released, showcasing the chorus of the new song, Black Heart. Over the following days, other clips teasing the song were released as well. On June 11, a mysterious poster was posted online. The poster showed eye close-ups of two men and women, along with the date 28 June 2018 and UNB's logo. It was later revealed to be the release date of the group's second mini-album. Later, on June 18, conceptual photos were released, which revealed the identities of the faces in the poster:
Jungha
Hangyul
Jueun
Anne
All except for Jueun had participated on alongside UNB. The photoset's title included the term "Performance feat.", indicating their participation in the album's upcoming promotion cycle. On June 28, the EP Black Heart was released, along with the music video of the lead track with the same name.
