In 1928, the Polish government decides to use 10% of its revenues to fund a secret high tech research centered focused on radio and the development of a next generation fighter.
1932: Polish radio researchers develop radar.
1933: Polish aircraft engineers develop Project 33 (P.33), an all-metal, single-seat, low-wing monoplane. The aircraft is equipped with a 1,500 hp engine and has maximum speed of 650mph, operational radius of 700 miles, service ceiling of 30,000 feet. It is armed with 4, 7.92 mm machine guns.
1934: P.33 fighter plane goes into production and 150 a year are ordered. Poland is also producing a medium bomber.
1935: Polish military secretly deploys a radar early warning system.
1939: Poland has 750 P.33 fighters at the beginning of war and a secret radar early warning system that Germans know nothing about.
In the opening moments of the war, the Luftwaffe encounters heavy losses as Polish P.33 guided by the radar network intercept German attempts to bomb cities and interdict supply lines. Problems with establishing air superiority cause Germans to lose momentum on the ground as their movement becomes increasingly difficult thanks to constant Polish air attack. Supplies such as food and fuel are running dry as Poles interdict German supply lines.
The first and biggest battle of the war, at Bzura, is a German failure thanks to air cover by the P.33s working in tandem with Polish medium bombers that decimate German armor and artillery positions. The Germans lose half of their armor, 200 of their aircraft, and suffer heavy casualties.
By third day of the war, it becomes increasingly clear to the Germans that they won't be making any progress against Poland very soon.
Heartened by Polish strength, the French decide to move against Germany. England commits to sending supplies to Poland under escort of lead by King George V battleship two cruisers and several destroyers.
Seeing unexpected problems in Poland, Stalin decides not to invade and bide his time until things clear up.
Meanwhile, facing a two front war, Germany collapses.
EDIT: Sorry, but there is an error here. The speed was supposed to be in kilometers, and similar to that of the German ME 109.
EDIT 2: Alteration in the scenario: After doing some research, I've learned that Poland had a plane with a 1,100 hp engine in 1937. No need to design anything at all, after all. They also had the industrial capacity and the money to make about 300 of these a year.
So the following now applies: they make 300 a year since 1934 of the PZL P.11. This gives them 900 of these planes by 1937, when they switch production to the newer version with a 1,100 hp engine, and make 600 of them by 1939. 1,500 fighters wipes out the Luftwaffe, given historical kill ratios.