At the risk of answering yet another let's-bash-the-nazis type question...
Whatever public claims he made, Hitler was always very big into destruction and, in fact, he "suggested" to the high command that Warsaw be bombed relatively early in the campaign which only lasted about three weeks. The high command refused to do this, having no military need to do so.
Unfortunately, this situation changed when, completely defeated, the Poles deliberately moved an entire army group into Warsaw by the order of their supreme commander, Marshal Rydz-Śmigły, who then announced that they would turn Warsaw into a "fortress" and defend it to the death.
The German army was then faced with a perplexing problem. To attack the "fortress" and turn it into a war zone, as the Polish army apparently wished to do, would have resulted in enormous civilian casualties since there were something like 1 million people living in the city, none of whom had been evacuated. Instead the Germans chose to encircle the city and invest it, essentially deciding to try to starve it out.
According to the German claims, After about a week of this course of action, it became clear that the city could hold on for some time, which, for political reasons (such as the advance of the Red Army), was undesirable. Therefore, the German army shelled several important militarized supply depots on the perimeter of the city and the Luftwaffe attacked several key supply depots inside the city. These were civilian buildings that the Polish army had converted to military use. These attacks occurred on the 25th and 26th of September. Two days later, on the 27th, the Poles surrendered the city.
The Poles tell a completely story. According to them the Germans bombed Warsaw from the very beginning of the campaign (September 1) and wantonly bombed targets indiscriminately, naturally preferring to attack schools, orphanages, churches and hospitals, etc.
Which version is true?
On September 11th, 1939, the AP reported on the Warsaw situation, the Poles claiming they had driven the Germans back and that the German air raids were "having no effect on them". The Germans claimed that they had withdrawn from certain suburbs because otherwise Polish shelling would have caused unnecessary destruction and they were adopting a policy of encirclement.
On the 13th the Boston Globe repeated the Polish bureau in Budapest's claim that Germans were shelling the city and listed military casualties as being 16 men that day, and there being "only 3 air raids".
The New York Times repeated the Polish bureau in Budapest's claim that the German's were constantly bombing Warsaw.
The Boston Globe on the 16th reported that the German policy was to encircle (not attack) Modin and that the advance to Warsaw was still in progress.
The New York Times dated 17 September 1939 published two photos of bombings, one a crater in a street, the other an apartment building.
On the 17th the Boston Globe reported that Germany had issued an ultimatum by dropped leaflets that the city must surrender or face military action.
On the 18th, Radio Warsaw announced that an emissary had been sent to "negotiate" with the Germans.
On the 19th, Warsaw reported that the center of the city was shelled, but no specific damage listed.
On the 20th, Warsaw Radio claimed that Warsaw had been bombed all day long causing "several thousand" casualties and destroying the cathedral.
Having read numerous press reports along these lines my assessment was that the truth lies somewhere between the versions. While certainly some bombing and shelling seems to have occurred before the 25th, it appears to have been relatively minor and mostly confined to the suburbs. It appears both sides exaggerated somewhat, but the Polish bureau in Budapest was guilty by far of making wild exaggerations.
I think the two photos in the NYT from the 17th showing a crater in the street and ruined apartment building is telling. Obviously if the German's had been doing serious bombing, there would have been much more dramatic photos than a crater in a street and a small ruined apartment building.