Don't know about developer APIs, but you could just use Google flight search. https://www.google.com/flights. You can easily issue GET calls with the search terms in the param or using the POST endpoint which takes some sort of request body that looks like it could be reverse engineered in an hour or so.

The GET to the resource looks like:
GET https://www.google.com/flights/#sear...4;q=SFO+to+LAX

and the POST:
POST https://www.google.com/flights/rpc
body:
[,[[,"ss","[,\"502464008F768.A465AA8.1B000DF3\",,6,{\"1\":[[,[\"SFO\"],[\"LAX\"],\"2014-09-13\"],[,[\"LAX\"],[\"SFO\"],\"2014-10-14\"]],\"18\":[[,3,1]]}]","241042555577193",20]],[,[[,"b_al","fa:53"],[,"b_ahr","fa:s"],[,"b_al","fs:133"],[,"b_ahr","fs:s"],[,"b_lr","13:199"],[,"b_lr","5:2"],[,"b_lr","6:76"],[,"b_ar","true"],[,"b_lr","12:0"],[,"b_pe","502464008F768.A465AA8.1B000DF3"],[,"b_qu","0"],[,"b_qc","3"]]]]