Kouya no Kotobuki Hikoutai Gaiden: Oozora no Harukaze Hikoutai
Japanese title: 荒野のコトブキ飛行隊 外伝 大空のハルカゼ飛行隊
Alternative titles: The Magnificent Kotobuki Side Story: The Harukaze Squadron of the Great Sky, The Kotobuki Squadron in the Wilderness Side Story: The Harukaze Squadron of the Great Sky, The Harukaze Squadron of the Great Sky
Type: ONA
Source: Original
Episodes: 12
Status: Finished Airing
Aired: Apr 10, 2019 to Feb 29, 2020
Duration: 14 min per ep
Rating: PG-13 - Teens 13 or older
Studios: WAO World, GEMBA
Producers: Digital Frontier, Bandai Namco Entertainment, Bandai Spirits
Genres: Action, Adventure
Themes: Military

A short spin-off anime released on "Kouya no Kotobuki Hikoutai: Oozora no Takeoff Girls!" game app and YouTube. 

(Source: AniDB)