Parijat Gaur wrote:........why is Russia developing Su-34 and Su-35BM simultaneously? I mean, Su-35 seems to be a very capable strike fighter in itself with large payload and good range. ............
The Su-34 is a cold war product and Su-35BM is free market product. The Su-34 was/is designed and produced to replace the Su-24 (which incidentally also has a side by side seating configuration....this is done to reduce duplication of certain instrumentation). Just like the Su-27PU (which became the Su-30), the soviets chopped up another series produced Su-27UB (added elements like the Canards from Su-27K...which becomes the carrier borne Su-33 and land based Su-35) and made the Su-27IB out of it. The Su-27IB was later re-designated as Su-34.
After the fall of the Soviet Union its every man/design house/production house for himself/themselves. So RSK Mig or MAPO made the Migs but Sukhoi OKB designed the Sukhois but the single seater Su-27 were made by KNAAPO (Komsomolsk-upon-Amur Aircraft Production Association), -27UB and Su-30K are made by IAPO (Irkutsk Aircraft Production Association) and Su-34 is made by NAPO (Novosibirsk Aircraft Production Association). Engines are designed by NPO Saturn but they are made by UMPO (Ufa Motor production association) for IAPO and MPP Salyut for KNAAPO. Then there are two radar mfg. Phazotron NIIR who make the Zhuk and Thikhomirov NIIP who make the Zaslon/Bars.
Thanks to MBA types there is now product differentiation and market segmentation. Sukhoi OKB in cooperation with IAPO and NPO Saturn in cooperation with UMPO with NIIP come up with a custom designed platform that "looks" like the Su-30 and it called the Su-30MK....which is MKI, MKM and MKA (the Indian Line) and Su-30 for Vietnam (?). The same Sukhoi OKB in cooperation with KNAAPO, MPP Salyut, NIIR come up with the Su-30MK....which is the MKK, MK2 and the MKV (the Chinese Line), Su-33 and Su-35. NPO Saturn + UMPO makes the first production TVC called the AL-31FP for the MKI. MPP Salyut without Saturn's blessing (or so they say) develop something called AL-31FN and sell it to China for their J-10. Both companies make the baseline AL-31F. Now they are developing the -31FM1 for the Su-27SM.
Now after the MKI was inducted the world realized that the MKI just happen to "look" like a Su-30 but its a completely different beast. IAPO now has a "superior" Su-30MK compared to Su-30MK offered by KNAAPO. That's where the Su-35BM comes in. It's basically KNAAPO evolution of Su-27SM > Irbis PESA + AL-41F1A (117S) TVC - canards = Su-35BM.
And sometime in 2000 Sukhoi + Mig + all production companies + others were merged into one outfit called Sukhoi AVPK. The Su-35 is primarily meant for domestic upgrades. I am not sure if PLAAF will see anything that sophisticated given their penchant to copy.