Instituted by Hermann Göring on 30 January 1941. It was awarded in Bronze (20), Silver (60), and Gold (110) with an upgrade to include diamonds possible. If you flew more, special pennants (instituted on 29 April 1944) suspended from the clasp indicated the number of missions obtained in a given type of aircraft - depending on the aircraft they required from 250-500 missions (see below). 

Front Flying Clasps were issued for missions completed in the following Luftwaffe aircraft:

• Day Fighters (Jagdverbände) - pennant for 500

• Night Fighters (Nahnachtjagdverbände) - pennant for 250

• Long Range Night Fighters (Fernnachtjagdverbände) - pennant for 250

• Heavy Fighters (Zerstörerverbände) - pennant for 400

• Air to Ground Support Fighters (Schlachtverbände) - pennant for 400

• Bombers (Kampf- und Sturzkampfverbände) - pennant for 300 and 400 respectively

• Reconnaissance (Aufklärungs- und Seenotverbände, Wettererkundungsstaffeln) - pennant for 250

• Transport and Glider (Transport- und Luftlandeverbände)  - pennant for 500

I can't help but to mention my favorite ww2 pilot here - Hans-Ulrich Rudel who flew 2530 combat missions. He got clasp in gold with diamonds with a "2000" pennant. For comparison US bomber crews were sent home after surviving 25 missions.