Titanic was a conservatively designed ship. There wasn't anything about her that was new, just bigger. She was designed for efficiency in the context of size in mind (For a more in-depth explanation of the design philosophies and comparisons to other ships, I recommend On a Sea of Glass; The Life and Loss of the RMS Titanic)

So, was the Titanic considered unsinkable? Yes.

In 1910, the White Star Line published this bit in regards to the upcoming sisters;

"...as far as it is possible to do so, these vessels are designed to be unsinkable."

I personally don't put much stock in the defence that ships at the time weren't advertised as unsinkable due to the qualifying statements. Being almost unsinkable is obviously not what the readers are going to take away from statements like this. As we now know, Titanic lacked a double-skin and the bulkheads were too low and not watertight, so no, they didn't design them to be as close to unsinkable as possible. This was corrected in the 1912-1913 refit on Olympic and on Britannic before she sailed in 1915.

Captain Edward John Smith himself made claims about all modern ships back in 1907 after completing the Adriatic's maiden voyage.

"I cannot imagine any condition which would cause [Adriatic] to founder. I cannot conceive of any vital disaster happening to this vessel. Modern shipbuilding has gone beyond that."

Lusitania embarked on her her maiden voyage almost exactly 4 months later. She and her sister Mauretania were declared by the New York Times to be "as unsinkable as can be."

But to put the crown jewel on all the above; Philip Franklin, the Vice-President of the White Star Line said on the morning of April 15th, 1912 in front of the New York office to an anxious crowd awaiting the news of Titanic's fate,

"We place absolute confidence in Titanic. We believe the boat is unsinkable."

Around midnight on 15th/16th, he would go on to say;

"I thought her unsinkable, and I based my opinion on the best expert advice. I do not understand it."

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What made them claim Titanic as "practically unsinkable" is actually preserved. Here's a direct quote from The Shipbuilder Magazine issue titled "Olympic and Titanic" dated June 1 1911. 13 days before Olympic's maiden voyage.

"In the event of an accident, or at any time when it may be considered advisable, the captain can, by simply moving an electric switch, instantly close the doors throughout, practically making the vessel unsinkable."

This switch was on the bridge, so it would have been instantly accessible. In 30 seconds (allowing for time to react), the alarms would be sounded below and the doors closed. This would split Titanic into 16 watertight compartments. This, combined with her double-bottom, is what made them claim she would survive any conceivable circumstance. The problem is, the accidents they conceived of were groundings and ship-to-ship collisions. The former would affect the double-bottom but (theoretically) leave the inner layer intact while the latter would affect two, maybe three compartments.

To be fair, this line of thought is not without merit. In 1907, a White Star Line ship Suevic ran aground. They couldn't get her off entirely, so they literally blew her in half and took the stern (full of all the vital machinery) back to Belfast and rebuilt the bow. She would go on to serve until the second World War when she was scuttled.

In 1909, the Republic (another White Star Liner) was struck by a ship and took 38 hours to sink. The only deaths were those in the initial collision.

On top of that, the Olympic survived a collision with a warship in September 1911. Not only survived, but sailed from Southampton to Belfast under her own power for repairs.

So 3 incidents just within the White Star Line in the 5 years leading up to Titanic that seemed to validate the theory that double-bottoms and compartmentalization would be more than sufficient to at least give everyone time to evacuate. This is also why there weren't enough lifeboats.

They didn't conceive of 6 compartments being compromised by a side-swipe.

Again, another great answer! Thank you! It’s amazing to me that this is what you can remember “off the top of your head.” Can I ask what led you to study this so deeply?

I’m a metallurgist and the properties of the Titanic’s iron has been extensively studied. Specifically the affects of the carbon and nitrogen content on the microstructure as it relates to the ductile-to-brittle transition temperature. I believe the last I read (in school 10 years ago), was that the iron was standard for the time period and that the temperatures likely did not cause excessive embrittlement.

I love the scene in the movie where J.Bruce Ismay says, incredulously, “this ship can’t sink!” To which Mr Andrews replies “she’s made of iron, sir. I assure you she can, and she will.”

I learned a lot and I appreciate your responses. I have some new books to add to my Kindle list, thanks to you!