The outbreak of WW2 is delayed or else never occurs
While as another poster has pointed out that research into jet engines commenced and patents were taken out many years before the development and wide scale utilization of jet engines the impetus for getting jets 'across the line' and into service was national survival.
Practical rockets, computers, radar, atomic energy/weapons and jet engines? All these things were direct outgrowths of the the fight for survival by nations during WW2. It was the war that saw governments invest the money, resources and pool the intellectual talent needed to rapidly accelerate the development of all these technologies. (If nothing else getting shot at tends to make you reassess your immediate priorities.)
Without the war development of all the above would no doubt have continued but at a more sedate pace. In the case of jet engines there was already commercial investment in piston engine aircraft and commercial aviation in general during the inter war years. But with no commercial incentive to spur the investment of large sums of capital in jet engine R & D or in new plant and equipment the development of piston engines would have just continued on apace until they reached the point where they were starting to hit their performance limits. TheseAnd these limits were what turned the attention of the military to jet engines during WW2 in the first place.