What was the first heavy carrier-based bomber?

The Boeing A-3 Sky Warrior is recognized as the first heavy carrier-based bomber, designed specifically for the U.S. Navy. Introduced in the 1950s, the A-3 was a subsonic jet designed to be launched from aircraft carriers, with the capability to deliver both conventional and nuclear ordnance. Its design allowed for a spacious bomb bay and the capacity to carry a significant payload, which was essential for the Navy's strategic bombing missions during the Cold War.

In contrast, the Boeing B-17 and the Boeing B-29 were heavy bombers used by the U.S. Army Air Forces during World War II, and they were not designed for carrier operations. The Grumman TBF Avenger was a torpedo bomber, primarily utilized for anti-ship warfare, and while it was an effective aircraft, it did not fit the heavy bomber classification like the A-3 Sky Warrior does.

Therefore, the A-3 Sky Warrior stands out as the correct answer since it specifically was designed and operated as a heavy bomber from aircraft carriers, marking it as a significant milestone in naval aviation history.