He was the highest-scoring non-British Allied ace in the Battle of Britain, with 17 confirmed victories and one probable, all gained in a period of four weeks in September 1940.

František was a brilliant pilot and combatant but frequently breached air force discipline first in Czechoslovakia, in France and Britain.

In 1936, he completed training and was posted to the 2nd Air Regiment at Olomouc in Moravia, where he flew Aero A.11 and Letov Š-328 reconnaissance biplanes.

In July 1939, a party of Czechoslovak airmen including František and another future RAF ace, Karel Kuttelwascher, went to the Port of Gdynia to sail to France.

While Czechoslovak pilots were preparing for departure, they received an offer to join the Polish Air Force, to which some agreed.

František was made an instructor, flying obsolescent Potez 25, Breguet 19, PWS-26, RWD 8, RWD-14 Czapla and Lublin R-XIII aircraft.

On one patrol from Góra Puławska a fellow Czechoslovak pilot, Zdeněk Škavarda, was flying a Breguet 19 when he ran out of fuel, made a forced landing near German lines and was at risk of capture.

[1] For his Polish service František was awarded the Virtuti Militari 5th class and received the Cross of Valour four times.

In Beirut, a week later, the four embarked on a Messageries Maritimes cargo ship, the Theophile Gautier, which landed them at Marseille on 20 October 1939.

There are no official French records to confirm he flew combat missions during the Battle of France, though some older sources credit him with 11 victories: nine in the air and two on the ground.

On 6 September he shot down a Bf 109E of III/JG 52 near Sevenoaks in Kent, but František's Hurricane was damaged so badly that he had to make a forced landing near Falmer.

František, perhaps due to his clash with the Czechoslovak air attaché in Paris, chose to stay with his Polish colleagues.

As all pilots were valuable, a compromise was reached whereby František was allotted a "spare" aircraft so he could fly as a "guest" of the squadron as and when he wanted.

This made him one of the top scoring Allied fighter pilots of the Battle of Britain and on 20 September, King George VI decorated him with the Distinguished Flying Medal.

[5][1] František was the first non-British pilot to have a bar added to his Distinguished Flying Medal when he posthumously received one on 28 October 1940.

[11] Posthumously the president of Czech Republic, Miloš Zeman awarded him the Order of the White Lion 1st Class Military Division on 28 Oct.

[1] In 2017, the Czech Mint issued silver and gold commemorative coins (under the authority of Niue) paying tribute to Josef František.

He was also referenced by Polish rock band Elektryczne Gitary in the song "Pilot Josef František", as a part of their 2016 album "Czasowniki".