Hi Artik,
>I have a question: Is Harvard a type of SBD?
No.
The "Harvard" was built by North American. Its Navy designation was SNJ, its Army designation was AT-6.
The SBD "Dauntless" was built by Douglas. Its Navy designation (obviously :-) was SBD, its Army designation was A-24.
SBJ was the Navy designation for the North American B-25 "Mitchell", so despite its similarity this designation addressed a very different aircraft.
Vultee, on the other hand, did indeed try to establish trainer and combat aircraft as different versions of the same basic airframe. These were the P-66 "Vanguard" fighter (144 produced), the BC-3 armed trainer (1 prototype) and the BT-13 "Valiant" trainer (produced by the thousands).
Regards,
Henning (HoHun)