Mahou Shoujo Jihen

As a little boy, Sakura Hiromi once dreamed he could be a hero. Now...he's simply your everyday office worker, toiling away for his company. Though he longs for his childhood dream, it seems so far out of reach—until he decides to step up and save a child one fateful day. Suddenly, fantasies become reality as Hiromi finds himself transformed into...a magical girl?!
(Source: Yen Press)
Note: Includes 3 bonus chapters.