The Ju89 and Do19 were not viable heavy bombers. You'd need them to be like the B17 in terms of stretch, but understand too that the B17 didn't have to be what it became until at least mid-1942. Even the Halifax and Short Stirling (the Wellington was a two engine bomber like the HE111) were only operational in late 1941 and were significantly later designs with the resulting aerodynamic improvements over the 1934 German Ural Bomber designs.
But let's assume that the Do19 is designed around more powerful engines so isn't as compromised as the OTL version was to accommodate the weak engines available in 1936. Let's say its got potential to be as good as the B17 with the right engines. It replaces the Do17 in production by 1938 (phased out to retool for the 19) and prevents the Do217 from getting into production. It would also replace the Fw200 in production in 1940, as it fulfilled that role and the He177 would have taken that on itself had it been ready. It replaces some of the He111's production because its taking over part of its roles. It gets into production in 1939 and into operational service in early 1940. Overall that means fewer bombers, but due to its greater payload, range, and defensive armament it is not only more survivable, but versatile. There still is likely some overall loss due to the greater maintenance requirements of the heavy bomber vs. medium bombers. That probable preempts certain missions like the strategic bombing attacks on French aviation industry taken during the French campaign and night attacks on Britain before the French were defeated.
It however probably doesn't affect the outcome of the French campaign, as it suffers less losses than the Do17 would have. Tactical missions are taken on by the Ju88 and He111s, while the Do19 instead focuses on more operational attacks (rail yards, ports, supply depots, airbases). For the sake of argument it has the ability to carry 4x the bombs of the Do17 and 2x that of the HE111, if not more by strapping extra external bombs to it like the B17. I doubt it would have been that good by 1940, but for the sake of argument, let's say so. Come the BoB it carries out the same missions as the lighter bombers, perhaps with greater surviveability due to heavier construction and defensive armament than the Do17 or even Ju88, as the British did not have a working cannon on their fighters in any numbers until October IIRC. It can take a heavier payload and thus allows a maximizing of the limited escort resources, as there were far more bombers available IOTL than fighters and bombing missions were limited by fighter escorts.
That doesn't ultimately matter to the BoB though, as there are probably too few Do19s to make a difference to the outcome even if they inflict more damage per mission and take fewer losses; they will still be shot up and worn down by the campaign. They wouldn't really offer that much more advantage during the Blitz. They might be more versatile in the East though, but they offer no added benefit over a He177 working and being ready in 1942 and in fact detract by being less capable than the He177 would have been.
It doesn't change the outcome of the war or really affect the outcome of campaigns, though it changes things on the margins and gets the old Do17 out of service earlier. If the Do19 had been technically worth it I'd say its better than producing the Do17, 217, Fw200, Ju290, or the non-functional He177 (though not a working He177), but that requires ASB intervention due to the horridness of the Ural Bomber designs
