Doraemon: Nobita and the Kingdom of Clouds(1992)

Doraemon: Nobita to Kumo no Oukoku

During class, Nobita questions the teacher where heaven is located when he was being taught about clouds. Being laughed at, Nobita looks for proof that a heavenly place exists on the clouds, but was explained by Doraemon that it cannot exist scientifically. Disappointed, he was eventually helped by Doraemon to create their own kingdom of clouds. Little did they know that there are creatures who have been living on clouds for a long time.

(Source: Anime News Network)