Absolutely he was! In fact, there is no way to become a Buddha without becoming a Bodhisattva first. Numerous Mahayana Sutras attest to this. Without developing the immeasurable compassion of a Bodhisattva there is no way for a sentient being to become a Buddha.
As for the Theravada who do not regard the Mahayana Sutras as authentic Buddhavacana: the Pali canon still has many references to the past lives of the Buddha where he was a Bodhisattva. And they are not all part of the so-called Jataka tales either. See here for many examples.
You can find an example here - AN 5.196 - which is not one of the Jataka tales:
“When the Tathagata—worthy & rightly self-awakened—was still just an
unawakened bodhisatta, five great dreams appeared to him. Which five?
“When the Tathagata—worthy & rightly self-awakened—was still just an
unawakened bodhisatta, this great earth was his great bed. The
Himalayas, king of mountains, was his pillow. His left hand rested in
the eastern sea, his right hand in the western sea, and both feet in
the southern sea. When the Tathagata—worthy & rightly
self-awakened—was still just an unawakened bodhisatta, this was the
first great dream that appeared to him.
Emphasis mine.
And here is the original Pali:
“Tathāgatassa, bhikkhave, arahato sammāsambuddhassa pubbeva sambodhā
anabhisambuddhassa bodhisattasseva sato pañca mahāsupinā pāturahesuṃ.
Katame pañca? Tathāgatassa, bhikkhave, arahato sammāsambuddhassa
pubbeva sambodhā anabhisambuddhassa bodhisattasseva sato ayaṃ
mahāpathavī mahāsayanaṃ ahosi, himavā pabbatarājā bibbohanaṃ ahosi,
puratthime samudde vāmo hattho ohito ahosi, pacchime samudde dakkhiṇo
hattho ohito ahosi, dakkhiṇe samudde ubho pādā ohitā ahesuṃ.
Tathāgatassa, bhikkhave, arahato sammāsambuddhassa pubbeva sambodhā
anabhisambuddhassa bodhisattasseva sato ayaṃ paṭhamo mahāsupino
pāturahosi.
Emphasis mine again.
Here is another example from the Pali-canon which is not one of the Jataka tales:
“So it is, Ananda. So it is. Even I myself, before my Awakening, when
I was still an unawakened Bodhisatta, thought: ‘Renunciation is good.
Seclusion is good.’ But my heart didn’t leap up at renunciation,
didn’t grow confident, steadfast, or firm, seeing it as peace. The
thought occurred to me: ‘What is the cause, what is the reason, why my
heart doesn’t leap up at renunciation, doesn’t grow confident,
steadfast, or firm, seeing it as peace?’
Emphasis mine.
And here is the original Pali:
“Evametaṃ, ānanda, evametaṃ, ānanda. Mayhampi kho, ānanda, pubbeva
sambodhā anabhisambuddhassa bodhisattasseva sato etadahosi: ‘sādhu
nekkhammaṃ, sādhu paviveko’ti. Tassa mayhaṃ, ānanda, nekkhamme cittaṃ
na pakkhandati nappasīdati na santiṭṭhati na vimuccati etaṃ santanti
passato. Tassa mayhaṃ, ānanda, etadahosi: ‘ko nu kho hetu ko paccayo,
yena me nekkhamme cittaṃ na pakkhandati nappasīdati na santiṭṭhati na
vimuccati etaṃ santanti passato’?
And here is yet another one:
“Bhikkhus, before my enlightenment, while I was still a bodhisatta,
not yet fully enlightened, it occurred to me: “What now is feeling?
What is the origin of feeling? What is the way leading to the
origination of feeling? What is the cessation of feeling? What is the
way leading to the cessation of feeling? What is the gratification in
feeling? What is the danger? What is the escape?”
Here is another one:
‘Both the quotations you have made, O king, are accurate. But when he
spoke of Āḷāra Kālāma as his teacher, that was a statement made with
reference to the fact of his having been his teacher while he (Gotama)
was still a Bodisat and before he had attained to insight and to
Buddhahood; and there were five such teachers, O king, under whose
tuition the Bodisat spent his time in various places—his teachers when
he was still a Bodisat, before he had attained to insight and to
Buddhahood. And who were these five?
And another:
At Sāvatthī. “Mendicants, before my awakening—when I was still
unawakened but intent on awakening—I thought: ‘What’s the path and
practice for developing the bases of psychic power?’
Now, why did I emphasize "intent on awakening?" Because, this translation chose to use "intent on awakening" as a substitute for the original Pali which has this:
Sāvatthinidānaṃ. “Pubbeva me, bhikkhave, sambodhā anabhisambuddhassa
bodhisattasseva sato etadahosi: ‘ko nu kho maggo, kā paṭipadā
iddhipādabhāvanāyā’ti?
So I hope you can clearly see that according to both the Mahayana and the Theravada that the Buddha was an unenlightened Bodhisattva before he became the Buddha.
Now, was the Buddha as a Bodhisattva for the benefit of all? Here is a passage from a canonical Pali sutta describing the Bodhisattva Gotama being born into the world for the benefit of all:
“The Bodhisatta, the foremost jewel, unequaled, has been born for
welfare & happiness in the human world, in a town in the Sakyan
countryside, Lumbini. That’s why we’re contented, so wildly elated.
He, the highest of all beings, the ultimate person, a bull among men,
highest of all people, will set turning the Wheel [of Dhamma] in the
forest named after the seers, like a strong, roaring lion, the
conqueror of beasts.”
And from that same sutta here is the testament to his motivation:
He, seeing the utmost purity, will set rolling the Wheel of Dhamma
through sympathy for the welfare of many.
So, to sum up: have no doubt The Buddha was a Bodhisattva who manifested the Mind of Enlightenment - pure Bodhicitta - for the benefit of all before his awakening.