The Luftwaffe did actually launch a large scale air bombardment of Stalingrad before the ground forces of the 6th Army began their initial attacks on the city. From the 23rd to the 24th of August, Luftflotte 4 commanded by Generaloberst von Richthofen commenced aerial raids and bombardment of the city.

The fourth air fleet flew a total of 1,600 sorties and dropped 1,000 tons worth of bombs for the loss of only three aircraft. The aerial assault on Stalingrad was the most concentrated attack by the Luftwaffe on the Eastern Front.

The issue is that flattening the city by air assault is not the same as capturing and holding it against counterattacks and a determined Soviet resistance.

Sources: Stalingrad & The Second World War by Antony Beevor.