I really doubt this is Sky Girls considering the 2004 date in the corner and the fact that we have two versions here on Danbooru that predate the OVA.

They're not even in the sky.

This is pretty much mechamusume, which served as a precursor concept for striker witches; specifically Strike Witches Mechanized Armored Infantry Witches, or "tank witches".

Bucue said:

This is pretty much mechamusume, which served as a precursor concept for striker witches; specifically Strike Witches Mechanized Armored Infantry Witches, or "tank witches".

Kinda hard to call them "tank witches" since they were created by Fumikane Shimada long before Strike Witches was a thing. Call them precursors to what is now known as Strike Witches would be more accurate.

AdventZero said:

Kinda hard to call them "tank witches" since they were created by Fumikane Shimada long before Strike Witches was a thing. Call them precursors to what is now known as Strike Witches would be more accurate.

Sort of yes, sort of no. The charicter concepts and designs of mecha musume girls like this based off of various tanks are what inspired and serve as precursors the concept of ground based armored infantry witches in the strike witches series and it's world building, while the concepts and designs of mecha musume based off of air craft is what inspired the air infantry witches of Strike Witches.

In essence, while these Panzer girls aren't responsible for completely inspiring the entirety of strike witches; they are an important key component and precursor to one of the important elements and components of the entire strike witches series as a whole along side the air craft mecha musume.

Mind you that still dosen't detract the gravitas of their importance to the series, but it's just that the exact importance is a bit more specific and less generalized.