Features 3 biplanes flying among the clouds on the front and a 4th biplane flying on the back without any clouds.
Also features a painted brown handle and matching rim.
Though no marking are present, due to its style I strongly suspect this mug was manufactured by Otagiri.
Condition note: there is a small brown discoloration inside this mug on the rear upper surface and rim. I believe it may simply be brown paint that ran over from the rim which is painted brown. Please see the red arrow pointing to this in the 5th photo.