For the history, you wanna begin on hard difficulty to unlock O.S.L as fast as possible but there are 2 things that you don't have to start on hard :

• There are some missions like Scramble and Intercept that you can't get an A rank with those aircrafts, ennemies are too resistants and you'll fail if you can't beat them down fastly. So you'll need to do those missions twice.
• Honestly, I did one try and I given up on Night Raven... On Hard difficulty, THE X-49 IS IMPOSSIBLE TO DESTROY UNDER 4 MINUTES WITH THE SU-43, R 103 OR XFA-36A. Atleast on NTSC-U version only i think because when I completed 100% on my ps2 PAL version which is actually buged as fuck, I didn't know how to unlock Geopelia so I completed Night Raven on hard with an A with 1 of the 3 aircrafts I quoted above.

The best thing to do is to complete the history on normal with A rank everywhere to get XR-900 Geopelia and to unlock mission simulator as fast as possible and then do hard difficulty with XR-900 to unlock the O.S.L. and then you finish with the easy difficulty