As /u/DBHT14 points out, this can be a difficult question to parse.

But I'll put forward the German bombing of Wieluń as the first likely civilian bombing. Three waves of Stukas bombed the town in the opening minutes of the war. They destroyed the hospital and around 70% of all other buildings in the town, killing about 1,200 civilians.

The town had no military value. It also had very little industrial value. So at first blush it appears to simply be a pure terror bombing. There evidently were German recon flights a day earlier (before the war even started) that sighted a Polish cavalry unit in the area. The initial bombings also took place in a heavy fog reducing visibility. This has led to claims that it was an attack on a Polish military unit that simply had civilian collateral damage. Or that it was intended to be an attack on a Polish unit, that missed entirely and hit a civilian target instead. Or that it was an intentional terror attack on a purely civilian target.

Questions regarding intent and motivations mean we likely won't ever know who "targeting" civilians first. But there is little doubt that the Germans (intentionally or not) hit civilians first.

So maybe if we can parse out a bit more about what you are actually hoping to get out of an answer we can help a bit more OP. Because obviously as worded its a difficultly broad question.

Do you mean in the sense that non combatants were killed as the result of air strikes be it CAS or more strategic attacks?

Do you mean when one side first began going after strategic targets in population heavy non military districts of urban areas?

Do you mean began the deliberate targeting of population centers to destroy enemy morale and ability to support the economy?